We report a quantitative full field electron density mapping of nano-scale objects by coherent x-ray diffractive imaging (CXDI). The projected electron density maps of two specimens, a single AuNi nanocrystal and a cluster of multi Au nanocrystals, were successfully reconstructed using CXDI. It was found that the accuracy of the electron density mapping was higher for the multiple Au nanocrystal specimen than the single AuNi particle. The improvement was attributed to the interference between the x-rays diffracted by each particle, which makes the diffraction profile complex and enhances the uniqueness of image reconstruction. The findings of this research will be useful for the quantitative analysis nano and bio systems using CXDI.