Despite much recent interest, it remains unclear what determines the rate of evolution of gene expression. To study this issue we develop a new measure, called "Expression Conservation Index" (ECI), to quantify the degree of tissue-expression conservation between two homologous genes. Applying this measure to a large set of gene expression data from human and mouse, we show that tissue expression tends to evolve rapidly for genes that are expressed in only a limited number of tissues, whereas tissue expression can be conserved for a long time for genes expressed in a large number of tissues. Therefore, expression breadth is an important determinant for evolutionary conservation of tissue expression. In addition, we find a rapid decrease in ECI with the synonymous divergence between duplicate genes, suggesting fast divergence in tissue expression between duplicate genes.