Crystalline-orientation and pump dependent polarization states of Yb: Y3Al5O12 microchip lasers were observed experimentally. Linear polarization was observed at six crystalline orientations in the (111) plane relative to the beam propagation direction along the  crystalline axis of Yb: Y3Al5O12 crystal. The extinction ratio of the linear polarization decreases and the laser tends to oscillate at random polarization state at high pump power levels. Linear polarization states of Yb: Y3Al5O12 microchip laser was selected by the anisotropic spectroscopic properties of the Yb 3+ -ion in cubic Y3Al5O12 crystal. The random polarization oscillation at high pump power was caused by the strong thermal-induced birefringence and depolarization. © 2008 American Institute of Physics.