The effect of the waveguide and the quantum well-dot active region on polarization of transmitted light in laser-like heterostructures was studied with the polarimetry technique. The waveguide structures with quantum well-dots were demonstrated to have almost no effect on the transmitted radiation polarization when it is parallel or perpendicular to active region layers. However, in the case of intermediate angle of linear polarization, the transmitted radiation represents a mixture of elliptically polarized and unpolarized light. The largest degree of depolarization found was at 45° of linear polarization angle of the input radiation. Depolarization was found to increase with decreasing of number of QWD layers and reaches 23% for a single layer.