Aim To study on the effect of over-expressing acyl coenzyme A cholesteryl acyltransferase-1 (ACAT-1) gene in the formation of foam cells in three kinds of cells. Methods ACAT-1 gene were transfected into cultured human THP-1 monocytic cells, human embryonic kidney 293 (HEK293) cells and murine macrophages RAW264.7 cells. The over-expressing ACAT-1 gene cells were incubated with acetyled low density lipoprotein (ac-LDL) and stained with oil red O to detect the formation of foam cells. Results The formation of foam cells became easier from over-expressing ACAT-1 gene THP-1 monocytic cells and RAW264.7 cells than control. But no apparent changes were observed in over-expressing ACAT-1 gene HEK293 cells. Conclusion Over-expressing ACAT-1 gene accelarated the formation of foam cells in monocytic and macrophagic cell lines.