The aims of the study were to investigate the effects of inclusion of defatted yellow mealworm (Tenebrio molitor, YM) replacing fishmeal on the pellet qualities of extruded feed and the growth performance of largemouth bass (Micropterus salmoides). Three formulations were used in the experimental diets, with replacing fishmeal at level (wt%): 0% (YM0), 50% (YM50) and 100% (YM100), respectively. Extrusion trials showed that the expansion rate and porosity of the YM50 feed pellets were higher than that of YM0 (P<0.05). However, the expansion rate and porosity of the YM100 was significantly lower than that of YM0 (P<0.05). Furthermore, the expansion rate, hydration time and hardness of the investigated diets had a strong correlation with the water absorption index and pasting properties of the raw recipe formulations. A total of 300 largemouth bass with about 80 g of initial weight were randomly distributed into 12 tanks and fed with one of the three isonitrogenous and isoenergetic diets for 8-week. The feeding trials revealed that the feed intake of fish in YM50 group was increased, but it did not compromise fish growth performance. However, the YM100 group had a negative impact on the growth performance of the largemouth bass. The apparent digestibility coefficient (ADC)_d and ADC_e in the YM50 group were significantly higher than that of other groups (P<0.05). In addition, the hydration time, hardness, expansion rate, microstructure parameters of the investigated diets had a strong correlation with the ADC of nutrients. The present study showed that YM can be used to replace up to 50% of fishmeal in the diet formulation without compromising the qualities of feed and the growth performance of the largemouth bass. But fully replacement negatively affected the qualities of feed and the growth performance of the fish.