The mechanical properties of the fabrics woven by the jet loom are inferior to those of fabrics woven by the jet loom are inferior to those of fabrics woven by the rapier or the shuttle looms, although not only the same textile weave was produced but also the same weaving yarns were used. Furthermore, the fabrics woven by the jet loom indicate paper-like features. These drawbacks of the fabric woven by the jet loom would be caused by the marked deterioration of the yarn properties during the weaving process. In this work, the influence of weaving conditions of an air loom (AJL) on the mechanical properties of weft yarn is investigated. As a result, it is found that the maximum strength and the modulus of elasticity are decreased during the weaving process. However, It is found that there is a weak correlation between the decrease in the modulus of elasticity and that in the strength. Moreover. It is found that the decrease in the modulus of elasticity would be caused by bending strain of the weft yarn, and the decrease in the strength would be caused by the production of the stress concentration in the weft yarn during weaving.