The reliability and maintenance of fluid power systems impose the control of fluid contamination made by hydraulic filters. The standardized multi-pass test is currently the main tool for defining the efficiency of hydraulic filters. But, the test conditions of multi-pass test do not always match sufficiently with actual operating conditions. This may lead to unexpected difference between the filtration efficiency defined in the multi-pass test and the efficiency in real system. This paper presents a test unit used for the testing of hydraulic filters with variable flow. The aim of the research is to test the filtration efficiency under conditions, which are corresponding to real-life working conditions