The dependence of resistivity, carrier concentration, and grain size upon the impurity concentration and heat‐treatment temperature for heavily As‐ and P‐doped chemical‐vapor‐deposited polycrystalline Si is experimentally investigated, and compared with the resistivity and carrier concentration in single‐crystal Si. It is found that the resistivity and carrier concentration in polycrystalline Si are mainly determined by those in crystallite and the grain size which is dependent upon the heat‐treatment temperature. Evaluating the segregation of As atoms at grain boundaries from the experimental results of As diffusion into single‐crystal Si from polycrystalline Si, it is found that, even when impurity atoms scarcely segregate at grain boundaries, the carrier concentration in polycrystalline Si is lower than that in single‐crystal Si. To explain the phenomenon, it is assumed that impurity atoms are electrically inactive in the disordered region near the grain boundaries. Combining the above assumption and the experimental results, the width of the disordered region and carrier trapping density at grain boundaries are estimated to be 110 A˚ and 4×1012cm−2, respectively, for the samples heat treated at 1200 °C. It is suggested that As‐doped polycrystalline Si has a higher grain boundary barrier resistance than P‐doped polycrystalline Si.