Prostate cancer is one of the most common types of cancer in men all over the world. Chemotherapy, radiotherapy and surgery are conventional cancer treatment modalities. However, these modalities have many complications and side effects. Therefore, it is very important to support the treatment with compounds that will reduce the side effects and increase the effectiveness of the treatment. The aim of this study is to investigate the cytotoxic effects of N-acetyl cysteine (NAC), an antioxidant, on human prostate cancer cells. This study was performed on DU145 human prostate cancer cells. The cells were divided into 7 groups (control and 0.2, 0.5, 1, 2, 5 and 10 mM NAC groups). Cell index was determined using real time monitoring of electrical impedance in all groups. Apoptotic activity was determined using flow cytometry. Apoptotic activity was measured only 5 and 10 mM NAC groups. The cell index increased significantly at lower doses of NAC (0.2, 0.5, 1, 2 mM) but decreased at higher doses (5 and 10 mM). NAC significantly increased percentage of early and late apoptotic cells at 10 mM concentration. Percentage of necrotic cell significantly increased at 5 and 10 mM concentrations. Our data showed that NAC could significantly inhibit the proliferation of DU145 human prostate cancer cells in high doses. In addition, in this study for the first time, we report that NAC enhances apoptotic activity of human prostate DU145 cancer cells. These findings denote the antitumor properties of NAC.