At present, measurement systems for measuring diameter and roundness of steel-ball-type spherical parts have significant drawbacks, such as complexity and high cost. To overcome these challenges, we propose a method based on the Fresnel diffraction. It is found that the diameter and roundness error of a steel ball can be measured according to the light intensity distribution curve, based on the relation between the peak point and projection edge of spherical Fresnel diffraction, which is calibrated using the dot calibration plate. Experimental results show that the measurement uncertainty of the diameter and roundness error of a millimeter steel ball is less than 0.0031 mm (confidence level is 95%) and 0.0046 mm (confidence level is 95%), respectively, without considering the diffuse reflection and scattering of the steel ball surface. Our proposed system is easy to realize, cost effective, and does not need an additional optical system.