Abstract: Based on the GMS-5 satellite IR images, the meso α-scale convective systems (MαCS) were investigated in July and August 1993. The composite synoptic environment was examined for two active-MαCS periods and two null MαCS Periods in 1993 to diagnose the large-scale dynamical and thermal conditions for the development of MαCS over the Yellow Sea region. The distinct contrasts were revealed in the composite diagnostic fields between the active-and null-MαCS periods. The large-scale environment favorable for the MαCS development in the Yellow Sea region was obtained as follows: the low-level air with the high equivalent potential temperature, the conditionally unstable atmospheric stratification, the large amount of water vapor conveyed into the MαCS active region by the intense and steady low-level southerly jet stream, the mild ascending motion accompanied by the lower-tropospheric warm advection, and the upper-level divergence related to the diffluent flow on the right side of the exit of the subtropical jet.