A new model is developed to evaluate the theoretical blend irregularity of a two‐component blended yarn. By introducing the variation of the number of fibers per cross‐section along the yarn, the obtained expression can offer insight into the cause of blend irregularity. Two experiments show that the results from the new model have essentially the same changing tendency as experimental values. The value of the blend irregularity of one component is shown to decrease with the increase of its proportion and the blend irregularity also decreases in the case of finer fibers.