In order to investigate the effect of yarn feeding tension on the cover factor of weft knitted fabrics some experiments were carried out with the variables such as yarn feeding tension, loop density, yarn characteristics and weight of take-up mechanism of weft knitting machine. The experimental results were discussed by the method of analysis of variance. The conclusions were as follows; (1) the yarn characteristics were considered to be factors affecting on the cover factor of plain weft knitted fabrics. (2) It was considered that the yarn characteristics and loop density of plain weft knitted fabrics interact mutually and their interaction was considerd to be a factor affecting on the relaxation shrinkage. (3) As long as the yarn feeding tension and the weight of the take-up mechanism are below the experimental value of the feeding yarn, they do not affect on the relaxation shrinkage.