Diamondback moth, Plutella xylostella (L.; Lepidoptera: Plutellidae), is an important pest of crucifers worldwide. The extensive use of diamide insecticides has led to P. xylostella resistance and this presents a serious threat to vegetable production. We selected chlorantraniliprole (R_f) and flubendiamide (R_h) resistance strains of P. xylostella with resistance ratios of 684.54-fold and 677.25-fold, respectively. The R_f and R_h strains underwent 46 and 36 generations of lab-selection for resistance, respectively. Low cross resistance of R_h to cyantraniliprole was found. Cross resistance to chlorfenapyr, tebufenozid, and indoxacarb was not found in R_f and R_h strains. The P. xylostella ryanodine receptor gene (PxRyR) transcripts level in the R_f and R_h strains was up-regulated. Except for R_f34 and R_h36, PxRyR expression in all generations of R_f and R_h selection gradually increased with increasing resistance.Two resistant populations were field-collected from Guangzhou Baiyun (R_b) and Zengcheng (R_z) and propagated for several generations without exposure to any pesticide had higher PxRyR expression than the susceptible strain (S). In the S strain, PxRyR expression was not related to the resistance ratio. Gene sequencing found that the RyR 4946 gene site was glycine (G) in the S, R_f, and R_h strains, and was glutamate (E) with 70% and 80% frequency in the R_b and R_z populations, respectively. The 4946 gene site was substituted by valine (V) with the frequency of 30% and 20% in R_b and R_z populations, respectively.These results increase the understanding of the mechanisms of diamide insecticide resistance in P. xylostella.