That will depend on what the per diem is intended to cover. If it is for travel expenses only, it should be assigned to the Travel line on Schedule C (Form 1040), line 24a. If it is intended to cover meals alone, it should be assigned to line 24b. If it covers both meals and travel, you must make a reasonable allocation between the two lines.
Please keep in mind the 50% limitation on the deductibility of business meals.
For more information, please refer to:
IRS Pub. 463 Travel, Gift, and
Car Expenses
