A two-element f-.theta. lens used for a micro-electro mechanical system (MEMS) laser scanning unit includes a first lens and a second lens, the first lens is a bi-convex lens and the second lens is a bi-convex lens. At least one optical surface is an Aspherical surface in both main scanning direction and sub scanning direction, and satisfies special optical conditions. The two-element f-.theta. lens corrects the nonlinear relationship between scanned angle and the time into the linear relationship between the image spot distances and the time. Meanwhile, the two-element f-.theta. lens focuses the scan light to the target in the main scanning and sun scanning directions, such that the purpose of the scanning linearity effect and the high resolution scanning can be achieved.