Paclitaxel (also known as Taxol) is a well-known anticancer agent that blocks cell mitosis and kills tumor cells, and is often used in clinic to treat cancers. Despite the success of Paclitaxel, the development of drug resistance prevents its clinical applicability. Here, we screened an siRNA library against the entire human genomes using HeLa cells, and have find that lack of USP15 (ubiquitin-specific protease 15) causes Paclitaxel resistance. We also observed the decreased expression of USP15 in Paclitaxel-resistant human ovarian cancer samples. In addition, we have demonstrated that USP15 plays an essential role for stability and activity of caspase-3 during Paclitaxel-induced apoptosis. Thus, USP15 may be a candidate diagnostic marker and therapeutic target for Paclitaxel-resistant cancers.