The new prime minister of Rhodesia, Abel Muzorewa, announced today that he will be his own defense minister and that his white predecessor, Ian Smith, will serve as minister-without-portfolio in this country's first biracial Cabinet.

Muzorewa's decision to head the Defense Ministry and the sister ministry in charge of fighting the country's guerrilla war appeared strange for a bishop in the United Methodist Church with a reputation among his people for peacemaking and among his political colleagues for indecision.

Muzorewa's resolve to assume personal responsibility for the crucial war effort seemed designed to underline the transfer of power - limited though it is - from the white minority to the black majority. Only time will tell, however, whether Rhodesia's white military chiefs will follow Muzorewa's orders.

Another question mark is Smith's continued presence, which could undermine the bishop's attempt to convince skeptics of his power, particularly those in black Africa who regard Smith as a symbol of the white-minority rule that the new government here is meant to replace.

In all, the 17-member Cabinet contains five whites and 12 blacks, 10 from Muzorewa's United Africans National Council and two from the United National Federal Party that enjoys support from the military Ndebele tribe.

Two portfolios remained vacant because of a boycott by the Rev. Ndabaningi Sithole's Zimbabwe African National Union. Sithole claims last month's elections included widespread fraud. Party spokesman James Dzvova said police arrested at least eight party members during the night in raids on their homes and offices.

"This may be a question of trying to silence members of our party," he charged.

Asked to explain the arrests, a senior police spokesman said: "Certain investigations involving members of an internal political faction are at present being carried out."

Smith's position as minister-without-portfolio was no doubt aimed at fending off potential accusations that the wily politician still controls any important facet of government. The absence of a specific role for Smith also could be seen as way to facilitate his eventual departure from the Cabinet, which Smith has on several occasions implied would be a concession from him if the West recognizes the internal constitutional accord.

Some political observers note that Smith's position also raises the possibility that he will hover about Muzorewa as a powerful eminence grise . As such, he could brake the difficult decisions that are bound to come up - decisions for black advancement that the bishop will need to make to retain credibility among the people who elected him and to establish it among the estimated 12,000 guerrillas who oppose him.

Top officials in Muzerewa's party say they are not concerned about Smith's presence. "I don't think Smith is an important factor any more," said the new deputy minister of information, Ishmail Adam. "His presence without a portfolio shows he cannot affect the running of the government in any way."

The former prime minister's inclusion in the new Cabinet was widely anticipated here. Smith and the moderate black leaders who set up the biracial government agreed that Cabinet posts would be allocated proportionately to all parties winning seats in the new legislature. While Muzorewa could decide which ministries whites would head, it was up to Smith to nominate which members of his Rhodesian Front Party would occupy those positions.

This helps explain why two of the four other white ministers are well known for their right-wing beliefs - P.K. Van der Byl, who will be minister for posts, transport and power, and William Irvine, who will be minister of agriculture.

Whites also will maintain control of the Finance Ministry, where David Smith continues as minister, and the Justice Ministry, which remains with Chris Andersen.

To back up the new prime minister's appeal yesterday for cooperation from all groups, among the 24 ministers and deputy ministers is one who is colored, or mixed race, and one Asian. Ministerial posts went to all three subgroups of the large Shona tribe, from which Muzorewa draws his largest support.

Sithole's party won only 12 seats in the national legislature, compared to Muzorewa's 51. The two ministries to which his showing entitled him - health and roads - were allocated on a temporary basis, reflecting Muzorewa's apparent belief that Sithole eventually will relent.

"He's just playing to his galleries now," said one Muzorewa spokesman. "When he has exhausted all the constitutional means to discredit the election, he will see that he can achieve more by joining the government."