With the advent of the array-based methods, the observations of leaky modes from earthquake records and ambient noise are frequently reported. It is urgently needed to have an effective guide to the selection of certain leaky modes to put into practical inversion combined with other imaging methods. To this end the excitation of leaky modes for multi-layered models is theoretically investigated. By computing theoretical seismograms for different source mechanisms and different source depths with the discrete wavenumber method and the normal-mode summation method separately, the contribution of leaky modes to the resultant seismograms is qualitatively evaluated and the effect of the source depth for the same source mechanism is in detail examined. Further, we perform accurate computation of leaky modes for two models and categorize the leaky modes into PL and OP (organ-pipe) modes, whose distinctive properties are characterized both from the attenuation and from the eigen-displacements of these two types of modes. Also, these results could help explain very well the occurrence of certain leaky modes on the dispersion spectrum and be indicative of the reasonable choice of certain leaky modes to put them into practical inversion for P-velocity structures.