Consumer prices rose at an annual rate of 7.4 percent in August, while the purchasing power of an average hour's pay declined, the Labor Department reported yesterday.

Grocery store prices, the main cause of inflation the first half of the year, did not rise at all last month but prices of other goods and services picked up the slack.

Medical care costs rose 0.9 percent in August, and gasoline prices 1.1 percent. Prices of natural gas, which President Carter wants to deregulate, rose 1.4 percent.

Costs of owning a home rose 1 percent in August. For the last six months these costs have been climbing at an annual rate of 14.3 percent. Higher mortgage interest rates are a major factor, as the Federal Reserve Board raises interest rates in an attempt to fight inflation by making borrowing more expensive.

Overall, consumer prices rose 0.6 percent in August and were 7.9 percent higher than in August 1977. For the last six months, the Labor Department's consumer price index has been rising at an annual rate of 9.8 percent.

Because of inflation, an average hour's pay last month bought 0.4 percent less than in July, though still 0.3 percent more than a year ago.

Two administration policymakers took differing views of the inflation picture yesterday.

Barry Bosworth, director of the President's Council on Wage and Price Stability, said in a New York speech that inflation is "worsening." He said that the 0.6 percent rise in August consumer prices - although less than the 0.8 percent average monthly increase from January through June - was not "very encouraging."

Treasury Secretary W. Michael Blumenthal said that while the August rate of inflation is still too high, "we can expect a considerable moderation" during the rest of the year, according to United Press Internation.

John Layng, Senior price analyst for the Bureau of Labor Statistics, noted that when food is eliminated from the calculations, the inflation rate remains "stuck" where it has been for many months. Layng said he does not see the rate coming down soon.

When food is factored out of the consumer price index, inflation has been running for the past six months at an annual rate of 8.9 percent instead of 9.8 percent.

Economists think non-food prices are a better indicator of inflation. Food prices often rise and fall erratically, as they have this year.

Administration officials, who last January predicted a 6 percent rise in consumer prices this year, now are hoping to hold consumer inflation to 8 percent.

President Carter has promised a strengthened anti-inflation program that aides say he will deliver by mid-October. The plan is expected to contain "voluntary" guidelines on wages and prices, although organized labor has said such guidelines probably will not be acceptable.

Bosworth said yesterday the president has not yet decided whether to include guidelines in his program.

Federal Reserve Board Chairman G. William Miller, has ofen warned that unless other policies are put in place to help check inflation, continuing to tighten monetary policy could choke off the economy's expansion and trigger a recession.

The overall consumer price index stood at 197.8 percent of its 1967 average in August. That means a selection of goods and services that cost $10 in 1967 cost $19.78 last month. Although the Labor Department adjusts all the percentage changes for normal seasonal variations in prices, it does not seasonally adjust the overall index.

August was the second month in a row that food prices behaved moderately. - They declined in July, after rising 1 percent or more a month during the first six months of the year. The Labor Department said that, except for cereal products and dairy products, almost all broad foot categories declined in August.