Genetical genomics is combining of microarry technology and quantitative trait loci (QTL) analysis, mapping expression QTL (eQTL) in global level. It provides a novel procedure for revealing the molecular mechanism and regulatory network of complex trait. The conception and strategy of genetical genomics were brought forward by Janson and Nap in 2001. By now, genetical genomics has been applied on yeast, mouse, human and plant such as maize. These results indicated that the variation of expression level of genes is heritable complex trait. Expression QTLs are classified into cis-acting and trans-acting. Cis-acting eQTL is located on the same genomic region of the regulated gene, meaning the variation of mRNA level could be due to the polymorphism in the gene itself. Trans-acting eQTL is located on other genomic region, meaning other gene controls the variation of mRNA level. The integration of eQTL results, gene function annotation and statistic analysis will not only more precisely identify candidate genes controlling complex traits and the expression of related genes, but also construct regulatory networks for these complex traits.