Two large aftershocks of the 1999 Chi-Chi earthquake (M_L= 6.4 and 6) occurred to the south of the Chelungpu fault and in the China-Yi in Taiwan. Near-field seismograms estimated some source parameters of the two events. The near-field seismograms estimated some source parameters of the two events. The near-field displacement spectra can be described by Brune's ω-square model. The estimated values of stress drop (Δσ), apparent stress (σ_a), and scaled energy (E_s/M_o), of these two events varied from station to station with mean values of: Δσ= 991 bars, σ_a = 402 bars, and E_s/M_o= 1.0 × 10^-3 for the M_L 6.4 event; and Δσ= 831 bars, σ_a = 337bars, and E_s/M_o= 1.0 × 10^-3 for the M_L 6.0 one. This shows a high dynamic stress drop between these two events. The larger values calculated from the M_L 6.4 event indicated a higher percentage transformation of strain energy into seismic-wave energy comparing to the M_L 6.0 event.