Mutations of parkin are linked to early-onset Parkinson’s disease (PD). Here we show that stable transfection of parkin in the human dopaminergic neuroblastoma cell line SH-SY5Y markedly reduced the activities of both monoamine oxidase (MAO) A and B. The amount of DOPAC, which is produced during dopamine oxidation by MAO, was greatly reduced by parkin overexpression. Radioligand binding assays showed that MAO binding sites were decreased accordingly. Consistent with these, MAO-B protein level was much lower while the amount of MAO-A protein was not determined due to the lack of a suitable antibody. Co-transfection of either MAO with parkin in HEK293 cells did not significantly alter ubiquitination and degradation of each MAO. When we measured MAO expression by real-time quantitative RT-PCR, marked reductions were seen in SH-SY5Y cells stably expressing parkin, compared to the parental cells or a control line stably transfected with luciferase. In addition, parkin mutants defective in E3 ligase activity exhibited different effects on MAO expression. We found that parkin also significantly decreased mRNA levels of both MAOs in the mouse fibroblast cell line NIH3T3. Furthermore, MAO expression was significantly increased in human B lymphocyte cell lines derived from PD patient with homozygous, but not heterozygous deletion of exon 4 of parkin. Together, these results suggest that parkin suppresses MAO expression. This function may limit the production of reactive oxygen species generated by MAO in dopamine oxidation, and would thus be beneficial to the survival of dopaminergic neurons.