Microsieve silting principle is a type of semiquantitative contamination level testing method that has been used to measure the particle concentration (mg/L) of solid contaminants in the fluid of a hydraulic system. However, it is not enough just to provide accurate particle concentration data because the particle size distribution (PSD) of the solid contaminants in a fluid is required for contamination control of a hydraulic system. Based on the basic filtration equation, a new model was developed by using the microsieve silting principle to determine quantitatively the PSD of the solid contaminants in a fluid. This model was used to built a specific linkage between the volumetric flow rate passing through the fouled microsieve and the liquid viscosity, particulate contamination (N-1 and N-10, the number of particles larger than 1 pm and 10 mu m, respectively), the texture of the microsieve (the density and size of the pores), and the pressure drop across the sieve through such fitting parameters as the declining time constants, the initial flow rate, the flow rate reducing ratios, and the silting coefficients, which can be determined through experimental study together with data analysis and fitting. The PSD in the fluid can be derived from N-1 and N-10 according to the log-normal distribution of the PSD. Experimental studies on the influential parameter of the testing system show that the changes in the operational pressure had little impact on the PSD data from the derived model. Furthermore, two samples with different PSD5 and viscosities were studied to verify the precision of the PSD data obtained from the new model, and experimental results show that the PSD results derived from the new model are in good agreement with those measured from a liquid Automatic Particle Counter (APC) testing. (C) 2016 Elsevier B.V. All rights reserved.