Interior Secretary James G. Watt yesterday appointed University of Illinois Prof. David F. Linowes to head a commission that will examine government coal-leasing policies.

The commission is a creation of Congress, where Watt has been under attack for allegedly selling federally owned coal for far less than its fair market value. The five-member commission, authorized in the Interior appropriations bill that President Reagan signed Saturday, is to report within six months.

A report by the General Accounting Office concluded earlier this year that Watt sold coal leases last year for about $100 million less than their fair market value. Interior officials have denied that the government got less than it deserved, but skeptical members of Congress have tried to put a halt to any further coal sales.

The latest attempt came Wednesday, when the House Interior Committee voted to stop a scheduled sale in Montana and North Dakota next month. Interior officials contend the vote amounted to an illegal legislative veto, and plans for the sale are moving ahead.

Linowes' new assignment will be his second tour of duty under Watt. In January, 1982, he headed a commission that recommended changes in the government's royalties policy, aimed at halting a pattern of underpayments that the administration said was costing the government up to $650 million a year.