Effects of temperature, soil moisture level, flooding, and soil microflora on decay of root galls in club root disease of Chinese cabbage were examined in the laboratory. Number of days required for complete decay of root galls was 3 days at $32^{circ}C$ or higher, 12 days at $16{sim}20^{circ}C$ and 28 days at $8^{circ}C$. As soil moisture content goes up, root gall decay became faster resulting 3 days for complete decay under saturated moisture condition at high temperature of $32^{circ}C$, and 8 days under the same moisture level at $24^{circ}C$. Soil moisture effect was relatively low at $24^{circ}C$ compared to $32^{circ}C$. Stimulation of decay by soil flooding was not observed at $32^{circ}C$ but became apparent at $12^{circ}C$. Influence of soil microflora on root gall decay was negligiable. Based on these results, temperature appears to be the most important factor affecting root gall decay in soil. Root gall decay is thought to be affected more easily by other environmental factors under low temperature conditions. Maturity of resting spores of Plasmodioprora brassicae in root galls tended to increase as time prolongs during root gall decay. Density of the resting spores was lower in fresh root galls where their maturity was also low as compared to completely decayed root galls. Number of resting spores in completely decayed root gall was $6.5{ imes}10^{6}/g$ tissue and its maturity was over 95%.