The accurate and complete assembly of both haplotype sequences of a diploid organism is essential to understanding the role of variation in genome functions, phenotypes, and diseases¹. Here, using a trio-binning approach, we present a high-quality, diploid reference genome, with both haplotypes assembled independently at the chromosome level, for the common marmoset (Callithrix jacchus), an important primate model system widely used in biomedical research^2,3. The full heterozygosity spectrum between the two haplotypes involves 1.36% of the genome, much higher than the 0.13% indicated by the standard single nucleotide heterozygosity estimation alone. The de novo mutation rate is 0.43 × 10^-8 per site per generation, where the paternal inherited genome acquired twice as many mutations as the maternal. Our diploid assembly enabled us to discover a recent expansion of the sex differentiated region and unique evolutionary changes in the marmoset Y chromosome. Additionally, we identified many genes with signatures of positive selection that might have contributed to the evolution of Callithrix biological features. Brain related genes were highly conserved between marmosets and humans, though several genes experienced lineage-specific copy number variations or diversifying selection, providing important implications for the application of marmosets as a model system.