My father inlaw has been living with me a year and is on social security.can I cliam as dependent?

Yes, if you provided more than half of his support and he did not earn more than $3700 in 2011.
So unless he is saving from social security, you support must be more than that amount.

Agreed; his Social Security income is NOT factored in when determining support, so it is almost certain that he is your dependent.

Social security is not income but it does factor in determining your support.