[Aim]  To investigate the potential of using the wasp Aphelinus mali as a biological control for Aphis gossypii, an important agricultural pest, more specifically, the relative preference of Aphelinus mali for different Aphis gossypii instars. [Methods]  The leaf disc rearing method was used to determine the developmental duration, emergence rate, sex ratio, longevity, and hind tibia length of Aphelinus mali offspring parasitizing different Aphis gossypii instars in a laboratory. The behavior of female Aphelinus mali was observed and recorded for 30 min on leaf discs with 2nd and 4th instar Aphis gossypii. Dissection was employed to evaluate the number of eggs laid, and aphids consumed, by female wasps within 24 h. [Results]  Offspring of Aphelinus mali were able to complete their development by parasitizing all Aphis gossypii instars. However, parasitism of older Aphis gossypii instars was significantly less than that of younger instars (P<0.001), and both the developmental duration and hind tibia length of Aphelinus mali that parasitized older instars were significantly longer (P<0.001). The proportion of female offspring produced tended to increase with host instar, but this difference was not statistically significant (P=0.081). No significant difference was found in the adult longevity of Aphelinus mali (females: P=0.074; males: P=0.411). In choice tests, although female Aphelinus mali parasitized significantly more 2nd than 4th-instar aphids(P<0.001), no such preference was apparent with respect to predation (P=0.468). [Conclusion]  Parasitizing younger aphid instars resulted in shorter developmental duration and higher emergence rates for Aphelinus mali, whereas parasitizing older instars produced larger wasps with a higher proportion of females. Aphelinus mali preferred to parasitize 2nd over 4th instar Aphis gossypii. These results clarify the oviposition and feeding preferences of Aphelinus mali for different instars of Aphis gossypii, providing useful information for the mass rearing of Aphelinus mali and facilitating the application of this species as a biological control for aphids.