Although basal cell carcinoma (BCC) with a diameter of 20mm or less is in general recommended to treat with 4 or 5mm surgical margin, there is no appropriate guideline what millimeters around the tumor one must excise in case the diameter of BCC is more than 20mm. Nine cases of BCC greater than 20mm, which histopathological sections were sliced at the center of the tumor, were measured their smaller tumor free width of the lateral ends of the specimen through their 50 serial sections retrospectively. They were actually undertaken operation with a surgical margin ranged between 4 and 10mm. No tumor cell was present in the both lateral ends of the nine histopathological samples with a width of 2 to llmm tumor free area, though their surgical margin was variable. Then it was calculated that at least 4mm excisional margin was required to eradicate the tumor thoroughly histopathologically. Since the subclinical extension of BCC is asymmetric, it is supposed that, there is a possibility of local recurrence, unless the tumor is excised with 2 or 3mm safety margin. Therefore the adequate surgical margin for BCC more than 20mm was concluded as 7mm when one can get at least 3mm tumor free area. Moreover the correlation between histopathological types of BCC and the severity of subclinical extension was not significant, though morphealike BCC was expected to be widely extended. Since the clinical margin of morphealike BCC is ill-defined, one must mark the margin much wider than usual, and then 7mm surgical margin is indicated for any histopathological types of BCC.