Amino acid transporters are essential for the growth and proliferation in all living cells. Among the amino acid transporters, the system L amino acid transporters are the major nutrient transport system responsible for the Na+-independent transport of neutral amino acids including several essential amino acids. The L-type amino acid transporter 1 (LAT1) is over-expressed to support cell growth in malignant tumors. The double stranded RNA-mediated RNA interference (RNAi) analysis can be in a wide variety of eukaryotes to induce the sequence-specific inhibition of gene expression. In this study, we examined the effect of LAT1 short interfering RNA (siRNA) on cell growth using siRNA of LAT1 in the KB human oral squamous cell carcinoma. In the RT-PCR analysis and western blot analysis, the siRNA of LAT1 inhibited expressions of LAT1 mRNA and protein. The uptake of [14C]L-leucine was inhibited by siRNA of LAT1. In the MTT assay, the siRNA of LAT1 inhibited the growth of the KB cells in the time-dependent manner, indicating that the growth inhibition of KB cell by the siRNA of LAT1 is induced by the blocking of neutral amino acid transport mediated by LAT1. These results suggest that the transport of neutral amino acids including several essential amino acids into the KB human oral squamous cell carcinoma is mediated mainly by LAT1. Further, the LAT1 would be a new target for the inhibition of cancer cell growth.