A low-profile wideband aperture-fed microstrip patch antenna (MPA) under radiation of TM and TM modes is proposed with improved radiation patterns. Initially, the radiation characteristic of a rectangular MPA operating in TM mode is theoretically investigated. The results demonstrate that its E-plane sidelobe could be gradually reduced by decreasing the resonant frequency of TM mode (f). As such, a pair of parallel slots is etched out on the radiating patch so as to significantly decrease the f. After that, the MPA under an operation of TM mode is further studied. It illustrates that its radiation pattern has the maximum off broadside direction and can be transformed into the broadside radiation by enlarging the length of the parallel slots. In addition, the width of the radiating patch is progressively enlarged in order to reallocate the dual modes in proximity to each other. With these arrangements, a wide bandwidth with improved radiation characteristics is obtained for the antenna under dual resonance. Finally, a prototype aperture-coupled MPA is fabricated and measured. The simulated and measured results agree well with each other, thus demonstrating that its impedance bandwidth is dramatically extended to about 14.8% with a low-profile property of about 0.048 free-space wavelength in thickness. Besides, the antenna has achieved a stable gain varied from 7.0 to 9.9 dBi over the operating band. In particular, the E-plane sidelobe level of the proposed antenna has been successfully suppressed.