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The air borne dust deposited on the surface of photovoltaic module influence the transmittance of solar radiations from the photovoltaic module’s glazing surface. This experimental work aimed to investigate the effect of dust deposited on the surface of two different types of photovoltaic modules (monocrystalline silicon and polycrystalline silicon). Two modules of each type were used and one module from each pair was left exposed to natural atmosphere for three months of winter in Taxila, Pakistan. Systematic series of measurements were conducted for the time period of three months corresponding to the different dust densities. The difference between the output parameters of clean and dirty modules provided the information of percentage loss at different dust densities. The dust density deposited on the modules surface was 0.9867mg/cm 2 at the end of the study. The results showed that dust deposition has strong impact on the performance of photovoltaic modules. The monocrystalline and polycrystalline modules showed about 20 % and 16 % decrease of average output power respectively compared to the clean modules of same type. It was found that the reduction of module efficiency ( in case of monocrystalline and polycrystalline module was 3.55 % and 3.01 % respectively. Moreover the loss of output power and module efficiency in monocrystalline module was more compared to the polycrystalline module