Laparoscopic microwave coagulation (LMC) for hepatocellular carcinomas (HCCs) was performed on 26 HCCs in 17 patients. Contrast-enhanced CT (CECT) and MR images (T1-weighted imaging [T1WI], T2WI, gadolinium-diethylenetriamine pentaacetic acid [Gd-DTPA] T1WI) were obtained to determine changes over time. The irradiated center exhibited low to moderate intensity with surrounded high intensity (HI) on T2WI and Gd-DTPA T1WI. On T1WI, lesions showed four patterns of intensity: uniform HI (30.8%), arcuate HI (26.9%), mainly low with spot HI (30.8%), and isointensity to hypointensity (11.5%). Follow-up imaging at more than 170 days revealed isointensity to hypointensity on T1WI (96.2%) and reduced HI on T2-weighted imaging (T2WI) and Gd-DTPA T1WI. All lesions became less conspicuous and were reduced in volume. HCC shows time-related changes in signals and size after LMC. Identifying the irradiated lesion is necessary to estimate the adequacy of treatment by comparison with the pretherapeutic image.