  We examine the role of the microwave power in the linear polarization angle
dependence of the microwave radiation induced magnetoresistance oscillations
observed in the high mobility GaAs/AlGaAs two dimensional electron system.
Diagonal resistance $R_{xx}$ was measured at fixed magnetic fields
corresponding to the photo-excited oscillatory extrema of $R_{xx}$ as a
function of both the microwave power, $P$, and the linear polarization angle,
$\theta$. Color contour plots of such measurements demonstrate the evolution of
the $R_{xx}$ versus $\theta$ line shape with increasing microwave power. We
report that the non-linear power dependence of the amplitude of the
radiation-induced magnetoresistance oscillations distorts the cosine-square
relation between $R_{xx}$ and $\theta$ at high power.
