The structure of the plain tricot knitted fabrics underlies two-bar tricots knitted fabrics. The biaxial tensile properties of the plain tricot fabric are calculated theoretically from the structure of fabric and some of mechanical properties of yarn, applying the method developed by S. Kawabata.The tensile process is divided into two regions, i.e., the bending effective region and the stretching effective region. The border of these two regions can be given by critical stretch ratios 'id and AC2 where suffixes 1 and 2 indicate wale direction and course direction, respectively.The tensile properties in the two regions are analyzed by using a new model which is different in structure from single dembigh stitch. The model used for plain tricot stitch consists of needle loop and sinker loop, i.e., layer of front yarn and of back yarn. In this model are two yarn crossing points; one is of the front yarn layer and another is of the back yarn layer. It is assumed that these two crossing points are in contact with each other and do not seperate during deformation process before critical ratios, that is, in bending effective region, but, after the critical state, these two points are separated in proportion to 22. The other analytical method is same as that of single dembigh fabric. Yarn frictional properties between crossing yarns are introduced and the accuracy of the calculation is much more enhanced due to this improvement, compared with the theory of single dembigh fabric.