The lady beetle, NephusarcuatusKapur is one of the most important predators of spherical mealybug,Nipaecoccusviridis(News.) in Khuzestan province, Southwest of Iran.The development time of immature stages (eggs, larvae, prepupa and pupa), males and females and the voracity of the 4th instars larvae and adults of N. arcuatus on 1st instar nymph of mealybug were studied in incubator (30±1ºC, 65±5% RH, and 14:10 L: D photoperiod). The number of larval instars was determined based on Dyar`srule. The head capsule width increasedfrom 0.15±0.002 (1st instar larvae) to 0.33±0.002 (4th instar larvae) as the larval stages increased. According to our results, the head capsule width of N. arcuatus followed Dyar`srule, but a second degree polynomial equation (ln y=-0.040 x2+0.456 x -2.316) can more accurately describe it. The 4th instar larvae, adult male and female consumed 177.8±5.46, 58.7±2.96, and 86.5±7.42 1st instar nymph,respectively during 24 hours. The periods of egg incubation first, second, third and fourth larval instars, andprepupal and pupal stages were 3.92±0.02, 2.17±0.05, 1.29±0.03, 1.52±0.03, 2.57±0.05, 1.05±0.03 and 5.09±0.12 days, respectively. Female and male longevity was 113.88±6.3 and 86.08±6.29 days, respectively and females laying an average of 660.47±37.97 eggs.