Liver metastasis is one of the most important prognostic factors in lung cancer patients. However, current therapies are not sufficient. RNA interference provides us a powerful and promising approach for treating human diseases including cancers. Herein, we investigated the in vitro effects of PLK-1 small interfering RNA (siRNA) on human lung cancer cell lines and the in vivo usage of PLK-1 siRNA with atelocollagen as a drug delivery system in a murine liver metastasis model of lung cancer. PLK-1 was overexpressed in cell lines and in cancerous tissues from lung cancer patients. PLK-1 siRNA treatment inhibited growth and induced apoptosis in a concentration-dependent manner. To verify in vivo efficacy, we confirmed that atelocollagen was a useful drug delivery system in our model of implanted luciferase-labeled A549LUC cells by detecting reduced bioluminescence after an i.v. injection of luciferase GL3 siRNA/atelocollagen. PLK-1 siRNA/atelocollagen was also successfully transfected into cells and inhibited the progression of metastases. This study shows the efficacy of i.v. administration of PLK-1 siRNA/atelocollagen for liver metastases of lung cancer. We believe siRNA therapy will be a powerful and promising strategy against advanced lung cancer.