The Supreme Court ruled 6 to 3 yesterday that Iowa's unique method of taxing corporations complies with the Constitution, neither denying due process of law to multistate business nor impermissibly burdening interstate commerce.

Iowa taxes corporations solely on the basis of the share of net income attributable to sales made with the state. Out-of-state corporations say this discriminates against them.

In contrast, 44 other states that impose corporate income taxes use a formula that relates payroll, property and sales within their boundaries to the same three factors nationally. A 45th state, West Virginia, uses a two-factor property-payroll formula.

Under the Iowa formula, Moorman Manufacturing Co., a Quincy, Ill., agricultural supplier with a large Iowa operation, was billed for 22,6177 percent of its 1968 income of $97.7 million. Under the three-factor formula, it would have been billed for only 14.1088 percent, it said.

In the opinion for the court, Justice John Paul Stevens said that the Iowa law providing for the single-factor formula gave Moorman an opportunity to demonstrate the result was arbitrary. But Moorman made no such showing in the record, he said.

The record also showed that the existence of duplicative taxation as between Iowa and Illinois was speculative, Stevens continued. But even if there were an overlap, the court cannot accept Moorman's "argument that Iowa rather than Illinois was necessarily at fault in a constitutional sense," he said.

The prevention of duplicative taxation "would require national uniform rules for the division of income," but it is to Congress, "and not this court, that the Constitution has committed such policy decisions," Stevens said.

One dissenter, Justice William Brennan Jr., wrote that the Constitution's commerce caluse requires a state's taxation of interstate business to be fairly apportioned to the commerce carried on within it - and that the Iowa formula doesn't meet the requirement.

Another dissenter, Justice Lewis Powell Jr., joined by Justice Harry Blackmun, wrote that the Iowa formula is out of line with that of other states, may be "irrational . . . inevitably discriminates against out-of-state sellers . . . has not been justified on any fiscal or administrative basis" and consequently is invalid under the commerce calise.

Blackmun wrote separately that the three-factor formulas were "improvements and while not perfect, reflect more accurately the realities of the business and tax world." Now, with Iowa's "parochia'l' method upheld, 'There is little reason why other states, perceiving or imaging a similar advantage to local interests, may not go back to the old ways," he said.

Savings and Loan Associations

In an 8-to-1 decision also announced by Justice Stevens, the court ruled that the states do not have to treat federally chartered savings and loans associations identically with local thrift and home financing institutions and federal credit union in imposing taxes.

The court rejected a challenge by 34 Massachusetts federal S&Ls to a state law imposing an excise tax based on their net operating income.