Comparative genomics is one of the important research fields in genomics. By comparative genomic studies it was possible to establish a giant genetic system beyond one species which might be significant to tree. Comparative genomics in Salicaceae, Pinaceae, Rosaceae, Fagaceae and Hamamelidaceae had shown that the organization of genomes remained highly conserved over long evolutionary periods and extensive synteny or collinarity or microcollinarity existed in tree genomes. A total of 13,019 pairs of orthologs were identified between genes in Populus and Arabidopsis using the best bidirectional Basic Local Alignment Search Tool (BLAST) hits, with an average mutual coverage of these alignments equal to 93%; 11,654 pairs of orthologs had greater than 90% alignment of gene lengths. The progress of comparative genomics in tree species was reviewed and the prospect of this field was discussed, which might be useful to tree genomics in China.