The effect of mobile phone e-mailing on young people's social tolerance was investigated by analyzing survey data of high school students in Japan. The results show that mobile phone e-mailing had a positive promoting effect on the homogeneity and a negative effect on the heterogeneity of personal networks. This in turn had a negative effect on social tolerance toward others. Mobile phone e-mailing enables students to select homogeneous others as companions of communication and thus brings a bias of homogeneity into their personal networks. This homogeneity bias in turn had a negative effect on the development of social tolerance in the socialization process by reducing the chance of interaction with heterogeneous others.