Polystyrene (PS)/nylon6 (PA6)/carbon black (CB) composites were prepared in Haake rheometer. The factors influencing the electric conductivity of PS/PA6/CB composites were investigated. Results showed that CB particles only distribute in PA6 phase because of the strong polarity of PA6. The CB content in PA6 phase and the morphology of PA6/CB phase affect the electric conductivity of PS/PA6/CB composites. No matter the PA6/CB phase is continuous or dispersed, the electric conductivity of PS/PA6/CB composites increases with the increase of CB content in the PA6 phase; higher continuous degree of PA6/CB phase leads to higher electric conductivity. A new term, statistical average critical distance (SACD), was defined to explain the electric mechanism of PS/PA6/CB composites, which is a function of CB content in PA6. The SACD of PS/PA6/CB composites, in which CB content is 23 wt % in PA6, was quantitatively counted, which is about 54 nm. Keeping the CB content in composites at 5.67 wt %, the optimum mass ratio of PS/PA6 for the composites having the highest electric conductivity is 80 : 20, resulting from the competitive result of the earlier two factors (that is the CB content in PA6 phase and the morphology of PA6/CB phase). (c) 2006 Wiley Periodicals, Inc.