Activated ras genes are found in a large number of human tumors, and therefore are one of important targets for cancer therapy. This study investigated the antitumor effects of a novel single chain fragment variable antibody (scFv) against ras protein, p21Ras. The anti-p21Ras scFv gene was constructed by phage display library from hybridoma KGHR1, and then subcloned into replication-defective adenovirus vector to obtain recombinant adenovirus KGHV100. Human tumor cell lines with high expression of p21Ras SW480, MDA-MB‑231, OVCAR-3, BEL-7402, as well as tumor cell line with low expression of p21Ras, SKOV3, were employed to investigate antitumor effects in vitro and in vivo. Fluorescence microscopy demonstrated that KGHV100 was able to express intracellularly anti-p21Ras scFv antibody in cultured tumor cells and in transplantation tumor cells. MTT, Transwell, colony formation, and flow cytometry analysis showed that KGHV100 led to significant growth arrest in tumor cells with high p21Ras expression, and induced G0/G1 cell cycle arrest in the studied tumor cell lines. In vivo, KGHV100 significantly inhibited tumor growth following intratumoral injection, and the survival rates of the mice were higher than the control group. These results indicate that the adenovirus-mediated intracellular expression of the novel anti-p21Ras scFv exerted strong antitumoral effects, and may be a potential method for therapy of cancers with p21Ras overexpression.