A new mathematical model describing a plain knitted fabric is proposed in this paper. One major feature of this model is that the yarn in the fabric can be naturally curved with nonlinear mechanical properties. The new model is able to describe the dimensions and also the low stress mechanical properties of a plain knit. Based on an energy analysis, the inadequacy of the classic k-values is explained. Complex dimensional behavior and problems associated with relaxation of knitted fabric are discussed. A more precise prediction of fabric dimensions is possible by including the degree of set as one of the parameters. The paper also explains why most of the studies of knitted fabric dimensional properties in the past have been empirical.