On my husbands w-2 form , it showed federal taxable income and seperatly per-diem pay.. When filing our taxes, do we have to add the per-diem as icome. They did not take taxes out of the per-diem only the taxable income

Probably not. Even if represented as "per diem pay", the per diem is probably reimbursement for expenses incurred during work. If it was reported under an accountable plan, such reimbursement is NOT TAXABLE.

If it were taxable, it would be added to the wages in Box #1 of his W-2.

If you have doubts, call your husband's payroll department and ASK!