Cottony cushion scale, Icerya purchasi Maskell is one of the most important pests of citrus orchards in Mazandaran province and the east of Guilan province. The biology and dynamism of this pest was studied in orange trees, Citrus sinensis. Samples of 10 trees (from the end of twigs) were randomly taken weekly. The results indicated that the pest has two or three generations in a year in the west of Mazandaran province. In the first two years of this study, the first nymphal instars of the first generation were emerged in the late of May and the population peak was occurred in the early of July, while in the third year the population peak was observed in the middle of June. In the first two years of this study, in the second generation, the first nymphal instars were emerged in September and the population peaks was occurred in the late of September but in the third year the population peak of second and third generation were in the middle of August and in the early of October, respectively. The average fecundity was 645±108 eggs. The eggs were hatched into crawlers in 12±2 days and development time of first and second instars were 17±4 and 20±3 days, respectively. Cottony cushion scale was hibernated as second instar nymph, third instar nymph or adult female under leaves. The cardinal lady bird, Rodalia cardinalis Mulsant were scattered in citrus orchards, especially in gardens that did not receive periodic applications of herbicides and insecticides.