Clubroot disease, caused by Plasmodiophora brassicae, is one of the most damaging diseases of vegetable Brassica crops in the world. In the previous study, we identified one major quantitative trait loci (QTL), PbBo(Anju)1, and 4 minor QTLs against P. brassicae isolate race4. The 3 minor QTLs come from a clubroot resistant double-haploid (DH) cabbage line (Anju), and the remaining minor QTL (PbBo(GC)1) from a susceptible DH broccoli line (GC). The cumulative effect of these QTLs for clubroot resistance has not been examined so far. In this study, 10 lines with various combinations of the CR-genes were selected from the F2/F3 population to verify the independent and cumulative effects of each CR-gene. The disease index (ID) was rated on a scale of 0 (resistance) -5 (susceptible). As a result, the resistance parent (Anju) showed an ID of 0.2, whereas the susceptible parent (GC) had an ID of 5.0. The line having one major QTL PbBo(Anju)1 revealed moderate resistance (ID = 3.0). The lines involving minor CR-genes without PbBo(Anju)1 showed distinct susceptibility (ID = 4.1-4.6), whereas PbBo(Anju)1 plus one or some minor CR-genes brought out moderate resistance (ID = 1.5-3.0). These results indicate that PbBo(Anju)1 play a crucial role in the expression of clubroot resistance. Finally the line that was accumulated all of the five QTLs exhibited high clubroot resistance (ID = 0.2). Therefore, the result of this study indicated that clubroot resistance can be reinforced by pyramiding minor CR-gene.