The vertical gradient of normal gravity usually taken as -0.3086 mgal/m everywhere on the earth's surface. This value is obtained on the assumption that the shape of the earth is sphere and its entire mass is concentrated on its gravity center. The rigorous expression of the vertical gradient of normal gravity can be obtained from the normal gravity potential. In this case, the mathematical formulation is made in the spheroidal coordinate system on the basis of the fact that the shape of the earth is approximately an ellipsoid of revolution. The obtained results indicate that the vertical gradient of normal gravity ranges from -0.3083 mgal/m (high latitudes) to -0.3088 mgal/m (low latitudes).