There are several kinds of nonlinear processes in the interaction between laser light and plane plasmas. Starting from two-wave coupling equations of plasmas, we discuss the instabilities appeared in homogeneous plasmas. If there is damp, the instabilities may appear, and when the damp is so small that can be neglected, there exists only spatial gain. The analytic solution of inhomogeneous plasma with density varying almost linearly are obtained and the conditions for occuring instabilities are discussed.