New Mannich bases 8a-f containing nucleobases were synthesized horn 4-methoxybenzaldehyde (1) and 2 hydroxyacetophenone (5) by the reactions including chloromethylation, nucleophilic substitution with thymine and uracil, and Claisen-Schmidt reactions. These new Mannich bases were evaluated for in vitro cytotoxicity against five human cancer cell lines: SK-LU-1, Hep-G2, MCF7, SW480 and P388. The results showed that most of the tested Mannich bases showed inhibitory activity against five cancer cell lines. Among the synthesized Mannich bases, compound 8b showed most potent cytotoxicity against MCF-7 cell line with IC50 value of 7.85 ug/mL.