Diadromus collaris (Gravenhorst) is an important pupal parasitoid of the diamondback moth, Plutella xylostella (L.), a major insect pest of cruciferous vegetables worldwide. We studied the development of immature stage of D. collaris by dissecting parasitized hosts in the laboratory at 25 ± 1°C and 50–80% relative humidity. The results show that all immature stages complete their development within the same host in 11≈12 d. The egg is hymenopteriform and appears to be anhydropic. There are four larval instars. The first instar is transparent with a sclerotized rectangular chitinous head capsule and distinct mandibles. Head capsule of the second instar turns into more isosceles trapeziumshaped. The third instar looks similar to the second instar but proportionally much larger. The sclerization and dimension of the fourth instar increases significantly. The pupa is a typical exarate form and lacks a cocoon. All life history was fully documented with detailed photomicrographs. Our study will be useful for understanding the physiological interactions between D. collaris and P. xylostella.