Rotation of the linearly polarized light caused by spherical alkali vapor cell is studied theoretically and measured experimentally. The results show that the vapor cell has no effect on the polarization state of linearly polarized light when the light beams pass through the cell center, and the incident plane is parallel or perpendicular to the polarization orientation. While the other positions will produce a certain rotation, and the rotation angle varies with the positions. For the ideal vapor cell with the external diameter of 12 mm and thickness of 0.9 mm, the theoretical rotation is up to 0.08° when a position is off center of 0.7 mm. Therefore, the effect of polarization rotation should be considered, when using the spherical alkali vapor cell as a central element of the instrument.