Partial dislocations in nanocrystalline metals are introduced and a modified dislocation density formula for partial dislocations is established by x-ray line profile analysis theories. Effects of factors on the determination of partial dislocation density are discussed. From the correlation between the partial and perfect dislocations, partial dislocation density is simply quantitative characterized by drawing on the evaluation methodology of perfect dislocations. Dislocation densities of nanocrystalline nickel calculated from two different equations are compared additionally.