Porokeratosis is an epidermal keritinization disordercharacterized clinically by raised hyperkeratotic border and histologically by the presence of cornoid lamellae. Several clinical variants have been recognized and development of squamous cell carcinoma (SCC) in lesions has been reported in all variants. Risk of malignancy is associated with older age, longstanding duration and linear variants. We report a case of SCC arising from porokeratosis of Mibelli. A 80-year-old woman presented with erythematous scaly plaque for 13 years. Physical examination showed erythematous scaly plaque, 5 x 6 cm in diameter, on left knee. After diagnosis of Porokeratosis was made, isotretinoin cream, calcipotriol ointment, and oral acitretin was prescribed, but lesions did not reached complete remission and has wax and waned. Ulceration with mass formation occurred after 4 years of initial diagnosis. Tumor excision was performed and histopathological examination showed a squmamous cell carcinoma with keratination. Excised margin was negative for tumor cells and patient was follow-up for additional 6 months without recurrence of the cancer.