Downy mildew, caused by Hyaloperonospora parasitica, is a destructive disease of the Chinese cabbage (Brassica rapa ssp. pekinensis). A rapid resistance/susceptibility test for H. parasitica was established by the inoculation of a leaf disk test. Four conditions were tested and the optimal condition was found when the inoculated leaf disk was placed into dark conditions at $16^{circ}C$ for the first 24 hours after inoculation, followed by 5 days of light at $20^{circ}C$ and $23^{circ}C$ for 4 hours and 5 hours, respectively, dark at $16^{circ}C$ and $12^{circ}C$ for 3 hours and 12 hours, respectively, and a final temperature of $16^{circ}C$ for 24 hours. There was a good correlation between the resistance levels of leaf disks and the resistance of seedlings or adult plants, which indicated that testing leaf disks should be the preferred methods to predict the resistance of adult plants. Using this method, downy mildew resistance was investigated in a double haploid (DH) population at four developmental stages. The results showed that the genetic pattern, which was deduced from the DH segregation, was relatively similar, but varied slightly during plant development.