Possibile causes of yam deformation in knitting are listed in the introductory part. The experiment included the production of samples of plain jersey weft-knitted fabric on a single cylinder automatic hosiery machine of E6 gauge, using single and twisted cotton yams of 48 to 96 tex. The samples were knitted employing four different yam tensile forces at the entrance to the knitting system: 5, 10, 15 and 20 cN. Basic yam properties before and after the knitting process are analysed. The yam fineness, number of twists, breaking force, breaking elongation, work to breaking and breaking strength significantly change in the course of knitting.