Abstract: We start our research based on the Pwave velocities of upper, middle, lower crust beneath the main tectonic units in western China to study crustal density and composition. The predictions of crustal composition are through two aspects. On the one hand, we compare the measured velocities in the wild with the observed velocities in the laboratory of various common rocks in these crusts. On the other hand, We use Vp -ρ relationship to convert VP to ρ. To determine the uncertainties in the crustal densities, we perform a Monte Carlo analysis perturbing the density structure to compute the density correction. The results show that most densities of the crusts (crystalline) in western China except western Sichuan are greater than 2830 kg/m3 which is the average density of global. Besides, both the Pwave velocities and densities in upper, middle and lower crust of main tectonic units in western China are corresponded with intermediate—acidic in shallow metamorphic, intermediate and mafic rocks respectively. Therefore, upper crust is acidic, middle crust is intermediate, lower crust is mafic in composition.