To improve filling factor of planar microlens arrays, square-aperture planar microlens arrays with good performance was fabricated with photolithography and thermal ion-exchanging technology, their fill factors was 100%. Imaging characteristics of unit lens and region among the four adjacent lens units (be called corner area) of square-aperture planar microlens arrays were analyzed theoretically and experimentally. Imaging characteristics in corner area are reverse to those in focusing element lens, which is showed by ray tracing and MATLAB simulation. Results with image testing system show that a reversed real image of object is formed after the ray passes through unit lens, while an erect virtual image is obtained because of divergent light out from corner area, which is caused by different refractive index distribution in these two areas. Therefore, square-aperture planar microlens arrays achieve the function of focusing and defocusing. Moreover, with enough ion-exchanging time, the uniform refractive index region in the corner area is so little that a new-type gradient refractive distribution is formed which increases gradually out from the center.