A theoretical and experimental study on calculating dislocation destiny for 4H-SiC homoepitaxial layers has been carried out. There is some difficulty in measuring dislocation density if it is more than 106·cm-2. In the paper, a theoretical analysis is made about the effects of dislocation density on the results of X-ray diffraction, and the relationship of dislocation density and FWHM spread is obtained. Then the X-ray diffraction curves of 4H-SiC in ω-2θ with two different crystal faces are presented from which the density of dislocation is calculated. According to the result, the cause of dislocation origin is analyzed and the methods of decreasing dislocation density are proposed.