Alpha particle spectroscopy is a measurement method that has found applied applications in several fields such as measuring nuclear decay data, geological studies or measuring low levels of activity in the environment. In all these measurements, determining the value of detection efficiency is very important, so that without it, quantitative measurement will not be possible. Several factors affect the detection efficiency. The purpose of this study is to measure the changes in detection efficiency with respect to of alpha particle energy, sample to detector distance and detector size. For this purpose, the efficiency of detection for different energies of 241Am, 239Pu, 244Cm, different sample-to-detector distances (2 to 52 mm) and different detector sizes (450 and 600 mm2) were investigated. The measurement results showed that the detection efficiency is reduced by about 20 times by changing the distance from the source to the detector from 2 mm to 5.2 cm while efficiency is not dependent on the energy of alpha particles (maximum variation of 11%). Also, the difference in source count in two different times and for two different detectors was about 1 and 1.2 percent, which indicates the stability of the system.