A microstrip antenna structure is provided, which includes a substrate, a ring microstrip line and a signal transmission port. The substrate has opposite first and second surfaces. The ring microstrip line is disposed on the first surface of the substrate. The ring microstrip line has a short coupling gap ranged between 0.004λg and 0.06λg for forming a radiation bandwidth with high selectivity, where λg represents a guided wavelength of an electromagnetic wave in the ring microstrip line corresponding to a center frequency of the radiation bandwidth. The signal transmission port is disposed on the second surface of the substrate. The signal transmission port penetrates through the substrate and is electrically connected to the ring microstrip line.
