A quantitative method to determine the activity concentration of 226Ra in soil samples was established using high performance environmental gamma-ray spectrometry. In this method, a semi-empirical calibration procedure was developed for full energy peak efficiency calculation utilizing the elemental composition of the soil sample. Aatami software was used to deconvolute the 235U and 226Ra doublet at 185.7 keV and 186.2 keV, respectively, and to fit the baseline of the soil gamma-spectrum for the determination of 226Ra activity. The results indicated that the Aatami doublet deconvolution procedure provides a rapid and accurate analysis of a complicated spectrum in comparison with other cumbersome spectral interference correction methods. The study also compared the results with those obtained by radon progeny (214Pb, or 214Bi) measurements and found that the deconvolution method provided a more accurate 226Ra activity as it is independent of the error caused by radon diffusion. This error can be quite large since the amount of escaped radon gas through the sample container walls and sealing cannot be accurately quantified.