A study of Mn‐site substitution by Ni2+ and the effect of annealing time in polycrystalline bulk La0.7Sr0.3MnO3 in composition range (0 ≤ x ≤ 0.1) is presented. This study involves structural, electrical, magnetoresistance, and thermoelectric power (TEP) measurements. X‐ray diffraction was used to examine the structure for the as‐prepared and annealed samples, and it shows that there is no change in structure with annealing time. Electrical resistivity and magnetoresistance are strongly affected by annealing process, where there is a dramatic increase in resistivity with annealing time and an enhancement in magnetoresistance in different temperature ranges. Moreover, conduction mechanisms above and below transition temperature (Tms) are discussed, since, variable‐range hopping and small‐polaron hopping models are applied above Tms, in addition to some experimental relations below it. In addition, the TEP sign changes from positive to negative with doping and annealing.