Former defense minister Evangelos Averoff-Tossizza was elected today to lead the conservative New Democracy Party, which was ousted by the Socialists in the October Greek elections.

Averoff-Tossizza, 71, regarded as the representative of his party's hawkish right wing, is replacing former prime minister George Rallis, 63, who lost the party leadership post Monday in balloting by the party's parliamentary group.

A meeting of the 112 New Democracy deputies gave Averoff-Tossizza 67 votes. Rival candidates Constantine Stephanopoulos and Ioannis Boutos, both former ministers, lagged far behind with 32 and 12 votes respectively. All three candidates had stated a commitment to the renewal of the party.

Rallis' downfall reflected wide disapproval of his preelection handling of the party, which lost support from both the center and the far right.

Rallis had snatched the party leadership from Averoff-Tossizza by one vote in May 1980, when New Democracy founder and leader Constantine Karamanlis vacated the position by moving to the presidency. The narrowness of the vote generated a party split that affected morale.

Averoff-Tossizza's election represents a swing to the right for Greece's largest opposition party. The former defense minister was the only politician of standing to espouse the idea that a "bridge" could be established between the ruling colonels and civilian politicians during the 1967-1974 military dictatorship here. His appointment by Karamanlis as defense minister after the return to democracy was viewed as an effort to avert any pro-junta backlash during the critical initial period of civilian rule.

Averoff-Tossizza began his political career as a member of parliament in 1946, and joined the Karamanlis-led National Radical Union Party in 1956. He is the author of several novels, as well as books on Greek politics and history, including an account of the civil war period after World War II and most recently a two-volume work on the Cyprus problem.