An experiment was conducted to obtain the basic information about competitive ability of rice against weeds as affected by eco-geographic race and plant spacing. Generally, more weeds were grown at the plot of Japonica-type cultivar, (Nakdongbyeo) compared to Tongiltype cultivar (Seogwangbyeo). The degree of weed suppression was highest at the 10 ${ imes}$ 10cm plant spacing while no difference was observed between 30 ${ imes}$ 15cm and 40 ${ imes}$ (10 ${ imes}$ 10 cm) plant spacings in both cultivar;. In all treatments except the plot of 10 ${ imes}$ 10 cm plant spacing of Seogwangbyeo, singnificant amount of weeds was higher for no weeding plot compared to hand weeding plot. Grain yield loss due to weed competition was the highest at the 40 ${ imes}$ (10 ${ imes}$ 10cm) plant spacing and followed by 30 ${ imes}$ 15cm and 10 ${ imes}$ 10cm plant spacing, in order, for Seogwangbyeo. However, for Nakdongbyeo, the highest grain yield loss was obtained from the 30 ${ imes}$ 15cm plant spacing and grain yield was not affected at the 10 ${ imes}$ 10cm plant spacing. Rice grain yield was highly correlated with weed weight for both cultivars (Seogwangbyeo; Y = 5.44 - 0.0128 X, r = -0.929$^{**}$, Nakdongbyeo; Y = 4.96 - 0.0103 X, r = -0.934$^{**}$). To reduce grain yield by 50%, weed weight of 212.5g/sq.m for Seogwangbyeo and 241g/sq.m for Nakdongbyeo was needed, respectively. This result implied that Nakdongbyeo was more competitive against weeds compared to Seogwangbyeo.