Carrier transport property of nickel incorporated iron pyrite Fe1-xNixS2 (0 <= x <= 0.1) has been characterized using temperature-dependent resistivity and Hall-effect measurements in the temperature range between 25 and 300 K. The resistivity of the pyrites decreased as the doping concentration of nickel is increased. Hall effect measurements show that lightly doped samples of x = 0, 0.01, 0.02, and 0.04 are p-type semiconductors while the heavier Ni-doped sample (i.e., Fe0.9Ni0.1S2) is n type. Temperature-dependent Hall-effect measurements show that both p- and n-type imperfections simultaneously exist inside the synthetic Fe1-xNixS2 (0 <= x <= 0.1). The acceptor level is caused by a defect state of chalcogen-deficient (SCl)(2-) while the donor level is induced by the nickel dopant. Temperature dependences of resistivity, Hall coefficient, and Hall mobility for the nickel-doped Fe1-xNixS2 are analyzed. The compensation behavior inside the Ni-doped iron pyrites is discussed. (C) 2008 The Electrochemical Society.