Periodic microstrip-line electromagnetic band-gap (EBG) structures with improved bandstop behaviors are constituted by adding the backside aperture and widening the strip conductor regarding to the low- and high-impedance microstrip-line sections, respectively. Its characteristics are studied by using the finite-difference time-domain (FDTD) method. After our FDTD-derived results are independently verified by those from ADS for an initial EBG structure, extensive work is done to investigate the concerned bandstop behaviors versus strip/aperture widths and lengths in order to achieve a broadened and deepened frequency stopband.