From the data of 31 large Taiwan earthquakes and their aftershocks published by Hsu (1971, 1980, and 1985) and reported in the Earthquake Data Report (EDR) by USGS, several aspects of mainshocks and their aftershocks are studied. Results show that the magnitude difference (ΔM) between mainshock and the largest aftershock increases with mainshock magnitude (M). The linear relations of ΔM vs. M are different for two data sets. The difference of original times of mainshock and the largest aftershock more or less decreases with magnitude of mainshock. The departure of the M_s-m_b scaling of the mainshock from that of aftershocks is bigger for larger mainshocks than for smaller ones. The mainshock magnitude cannot be estimated from the logN-M relation from aftershocks and is actually larger than the maximum magnitude evaluated from the relation.