This paper is mainly interested in data for the employment of five high-tech industries in 1988 and 1994 to test whether the dynamic density concentration explains industrial concentration in counties of the U.S.. After this paper introduces land density and congestion costs, dynamic density externalities induce the positive effect on new emerging high-tech industries because they benefit the new firms' activities so that positive externalities, such as knowledge spillover, dominate high competition and high land rents. On the contrary, they cause the negative effect on mature high-tech industries because high competition shrinks firms' activities. The possible explanation for the different effects is that the industrial structures are different in the stages of industrial development.