We developed a new potent nonviral gene transfer method into mouse muscles in vivo named "electrosonoporation." We tried in this report to treat murine orthotopic hepatocellular carcinoma (HCC) by muscle-targeted mouse interleukin-12 (mIL-12) gene transfer using in vivo electrosonoporation. I.m. administration of the mIL-12 gene with electrosonoporation elevated serum IL-12 and IFN-gamma and significantly prolonged the survival periods with both growth inhibition of orthotopic HCC and inhibition of spontaneous lung metastasis. The IL-12 gene therapy reduced the number of microvessels and induced more Mac-1-positive cells into HCC. These results show that muscle-targeted mIL-12 gene therapy for orthotopic HCC using in vivo electrosonoporation is very efficient and is thus promising for further clinical trial.