We consider the resolution of the single frequency reverse time migration (RTM)method for extended targets without the assumption of the validation of geometric optics approximation.The resolution analysis, which applies in both penetrable and non-penetrable obstacles with sound soft or impedance boundary condition on the boundary of the obstacle, implies that the imaginary part of the cross-correlation imaging functional is always positive and thus may have better stability properties.Numerical experiments are included to illustrate the powerful imaging quality and to confirm our resolution results.This is a joint work with Junqing Chen and Guanghui Huang.