A new straight forward theoretical approach was carried out to calculate the total and full-energy peak efficiencies for point, thin circular disk and cylindrical sources of Nal(Tl) and HPGe well-type detectors. The approach depends on the accurate calculation of two important factors: the path lenght, d, the photon traverses within the active volume of a gamma detector, and the geometrical solid angle, Ω, subtended by the source to the detector at the point of entrance. these two factors are theoretically derived through straightforward analytical formulae. Furthermore, the attenuation of photons by the source container and the detector housing materials is also treated by calculation the photon path lenght through these materials. The total efficiency in the energy range 0.1 - 10 MeV, for an axial point source placed at different heights, above the bottom of a bare well-type Nal (Tl) detector, has been calculated and compared with the published values; indicate a good agreement at different energies with zero discrepancies between the measured and theoretical values.In addition, the total efficiency is also determined for a set of cylindrical sources with different volumes, the percentage of deviations between calculated and published efficiency values are less than 4% In case of geometrical efficiency, it depends on the geometrical solid angle, Ω, subtended by the source to the detector at the point of entrance. The solid angle is essential in all absolute measurements of the strengths of radioactive materials and to calibrate detectors. The solid angle of the detection geometry is high, so that more than 95% can reach the detectors surface. The methode of high-efficiency ϒ counting by means of well-type HPGe and Nal (Tl) detectors is widely used and has proved a powerful tool, particularly when low-activity, small-volume environmental samples are to be analyzed by ϒ-ray spectrometry. A new analytical calculations of the solid angle of Nal(Tl) and HPGe well-type detectors using axial, non-axial, thin circular disk and cylindrical sources is proved in this paper. The geometrical efficiency of a well-type Nal (Tl) detector, for an axial and non axial point source placed at different heights above the bottom of the crystal well have been calculated and compared with the values obtained by using the Monte Carlo method and with the values obtained by using previous method of calculation published in (2006) by Abbas. The percentage deviations between calculated efficiency values are less than (0.1%). In addition, the geometrical efficiencies of a bare well-type detector, for an isotropic radiating coaxial circular disk source lies outside and for a co-axial cylindrical source lies inside and outside the detector crystal well as a function of different values of the circular base radius has calculated and compered The comparisions with the previous treatments reported in the literature indicate that the present approachis useful in the efficiency calculations of such complicated gama-ray spectrometer and better than that used by previous published papers. Elliptical integrals are met with the mathematical treatment to solve the obtained mathematical expressions.However, by means of mathCAD program the integrals are computed to calculate the detector's efficiency with high accuracy.