The optical microresonators with high Q factor and ultra-small mode volume have attracted great attentions for applications in optoelectronic devices. For a coupled-microdisks and a single microdisk connected with an output waveguide, coupled mode characteristics resulting from the structure asymmetry are investigated by the finite-difference time-domain (FDTD) method and the Padé approximation. Mode field patterns and mode Q factors are calculated, and the relative energies of mode-field components with different angular wave numbers are estimated from the coupled mode-field distribution. For coupled microdisk resonator, the higher-order modes introduced by the mode coupling usually reduces the mode Q factor of the coupled mode greatly. For a single microdisk resonator with a radius of 1 μm and a refractive index of 3.2 connected with a 0.2-μm-wide output waveguide, a coupled mode at the wavelength of 1.52 μm is observed with the mode Q factor of 268 and a triangle mode-field pattern, which is the coupled mode between whispering-gallery modes TE9,1 and TE6,2 with a wavelength difference of 6.6 nm.