Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar-nonpolar transitions, was observed for the first time in BiNi1-xFexO3 (0.25 <= x <= 0.5). The low-temperature phase was found to have a polar structure (space group of R3c) with a Bi0.5(1+x)3+Bi0.5(1-x)5+Ni1-x2+Fex3+O3 charge distribution and short-range ordering of Bi3+ and Bi5+. The volume reduction upon heating that was induced by charge transfer between Bi5+ and Ni2+ decreased with increasing x because of the reduction in the amount of Ni2+. Simultaneous polar-nonpolar transition also contributed to NTE, and a composition-independent enhanced volume reduction of similar to 2% was observed.