The efficiency of air jet weaving machines and fabric properties are influenced by many factors wherein the twist and weft tension during picking greatly affect physical and mechanical properties of woven fabrics. Many scholars agreed that weft yarn twist and tension affects physical, mechanical and comfort fabric properties. But they did not consider the twist loss of rotor spun weft yarn during air jet weft insertion. Freedom of the filling to untwist during air jet weft insertion results in twist loss during weaving which affects fabric properties. Fabric samples were produced from 20 Ne, 16 Ne, 10 Ne (single) and 10 Ne plied rotor spun weft yarns by changing air pressure and weft tension in an air jet weaving machine. Physical and mechanical properties were investigated by using different testing equipments according to ASTM standards. The result shows that the weft yarns have significance twist loss at 95 confidence level and twist loss on the right side of the fabric is higher than the left side. We found that the twist loss percentage of weft yarns varies with weft count and air pressure. For example the twist loss percentage of 20 Ne, 16 Ne, and 10 Ne weft yarns are 6.63 %, 5.74% and 4.44 % respectively with same air pressure. The result shows that twist loss percentage of plied weft yarn is higher than single weft yarn of the same count and the twist loss of weft yarn during air jet weft insertion significantly affects fabric thickness, tensile strength, and air permeability though other fabric properties like tear strength, bursting strength and GSM did not show significance effect. Moreover, tension of the weft yarn during air jet weft insertion affects fabric GSM, tear strength and bursting strength. We found that at 2 bar main nozzle with 3 bar relay nozzles air pressure and PFT/B+ gives minimum weft breakage rate (0.56 breaks/1000picks) for insertion of 20 Ne weft yarn in 190 fabric width. Key words: twist loss, weft tension, air jet loom, rotor spun yarn, woven fabric, fabric properties.