The objective of this work was to clarify some vague points in pressure step test, understand the know-how of the test and develop a similar test to detect very small leaks. The setup was used to detect small leak rates limited to 0.035 l/h/m3 of the pipe volume. The test period was~40 minutes which makes the rate of temperature change constant throughout the test. A computer program was developed to simulate the experimental test and calculate the leak rate theoretically. The experimental and theoretical leak rates were compared to check the validity of the derived equation. The comparison between experimental and theoretical results showed that the program is predicting leak rates with a maximum error of 5%.