Thermoluminescence (TL) method was investigated for detection of radiation treatment of black pepper (Piper nigrum), irradiated to gamma ray doses of 1.0 kGy and 5.0 kGy. The TL response of the minerals isolated from irradiated samples was much higher as compared to that of un-irradiated control samples. For the normalisation of results the separated minerals were re-irradiated to a normalisation dose of 1.0 kGy and TL glow curve recorded a second time. By comparing the glow curves of irradiated and unirradiated samples, it was possible to separate the irradiated samples from unirradiated ones. For laboratories having re-irradiation facilities, more accurate detection was possible by finding the ratio of the areas of first and second (re-irradiated) glow curves (TL1/TL2), which was confirmed by comparing the shapes of the glow curves of irradiated and un-irradiated samples.