Purpose: The HSV1-tk gene has been extensively studied as a type of reporter gene. In hepatocellular carcinoma (HCC), only a small proportion of patients are eligible for surgical resection and there is limitation in palliative options. Therefore, there is a need for the develoopement of new treatment modalities and gene therapy is a leading candidate. In the present study, we investigated the usefulness of substrate, 2'-fluoro-2'-deoxy-1-β-D-arabino-furanosyl-5-[124/125I]iodo- uracil ([124/125I]FIAU) as a non-invasive imaging agent for HSV1-tk gene therapy in hepatoma model using small animal PET. Material and Methods: With the Morris hepatoma MCA cell line and MCA-tk cell line which was transduced with the HSV1-tk gene, in vitro uptake and correlation study between [125I]FIAU uptake according to increasing numeric count of percentage of MCA-tk cell were performed. The biodistribution data and small animal PET images with [124I]FIAU were obtained with Balb/c-nude mice bearing both MCA and MCA-tk tumors. Results: Specific accumulation of [125I]FIAU was observed in MCA-tk cells but uptake was low in MCA cells. Uptake in MCA-tk cells was 15 times higher than that of MCA cells at 480 min. [125I]FIAU uptake was linearly correlated (R2=0.964, p=0.01) with increasing percentage of MCA-tk numeric cell count. Biodistribution results showed that [125I]FIAU was mainly excreted via the renal system in the early phase. Ratios of MCA-tk tumor to blood acting were 10, 41, and 641 at 1 h, 4 h, and 24 h post-injection, respectively. The maximum ratio of MCA-tk to MCA tumor was 192.7 at 24 h. Ratios of MCA-tk tumor to liver were 13.8, 66.8, and 588.3 at 1 h, 4 h, and 24 h, respectively. On small aninal PET, [124I]FIAU accumulated in substantial higher levels in MCA-tk tumor and liver than MCA tumor. Conclusion: FIAU shows selective accumulation to HSV1-tk expressing hepatoma cell tumors with minimal uptake in normal liver. Therefore, radiolabelled FIAU is expected to be a useful substrate for non-invasive imaging of HSV1-tk gene therapy and therapeutic response monitoring of HCC.