Cesium (Cs-137) is a technically enhanced natural radioactive element TENORM. Cs-137 is presented out of nuclear accidents as an example of anthropogenic radionuclides. In this paper, the activity concentration of Cs-137 is estimated in fly ash samples with a NaI (Tl) detector (gamma spectroscopy). It has high efficiency for gamma measurements. Fly ash samples were collected from the electric thermal power station Al-Hartha at Basra city in the south of Iraq. About fourteen samples were collected, prepared, and measured. The Cs-137 activity concentration is found between 0 (BDL, blew detection limit) to 28±7 Bq kg-1 for the fly ash samples from electric thermal power station Al-Hartha. All measured values are lower than the worldwide background level (UNSCEAR recommendation 37 Bq kg-1). Also, the obtained Cs-137 activity concentration has no effect on the estimated effective dose rate, that be prophesied with the activity of Cs-137 and cosmic radiation. The Cs-137 contribution in the estimation of absorbed dose rate will discuss and calculation will present for the measured activity.