[Objectives]  The gut microbiota of insect play an important role in food digestion, development and environmental adaptation. This study aims to compare the functional diversity of the gut microbial communities of deltamethrin-resistant and -sensitive diamondback moth larvae. [Methods]  Gut samples from 1^st to 4^th instar larvae of deltamethrin-resistant and -sensitive diamondback moths were analyzed with Biolog-ECO. [Results]  AWCD values from younger instar larvae were all higher than those of older larvae and AWCD values for larvae of the same age were nearly identical. Thirty tested carbon substrates were metabolized by all gut samples with the exception of the carboxylic acid substrate 2-Hydroxy benzoic acid. The Shannon and McIntosh indices of sensitive larvae were higher than those of the resistant larvae. The Shannon indices of 1^st to 3^rd instar sensitive larvae were nearly the same, and were higher than that of 4^th instar larvae. Resistant larvae could be ranked in terms of the Shannon index as follows: 1^st instar> 2^nd instar>3rd instar>4^th instar. In both sensitive and resistant larvae, the gut microbial McIntosh index was highest in the 1^st instar, with 2^nd to 4^th instars having similar McIntosh index values. [Conclusion]  The physiological activity of larval gut microorganisms in younger larval instars was higher than that in older instars. The functional diversity of the gut microbial community differed between deltamethrin-resistant and -sensitive diamondback moth larvae.