This paper describes the analysis of lanthanum nickel oxide (LaNiO3) formation process during heating of a precursor prepared via a mechanochemical (MC) route. To study the effect of the MC treatment on the solid-state reaction of the precursor, two kinds of precursor were prepared: one was a simple mixture of lanthanum hydroxide (La(OH)(3)), nickel hydroxide (Ni(OH)(2)), and sodium chloride (NaCl); the other was the mixture obtained by mechanochemical treatment of the simple mixture in a high-energy planetary ball mill. The MC-treated precursor consisted mainly of La(OH)(3), Ni(OH)(2), and NaCl, as did the simple mixture. However, the MC- treated precursor contained relatively low crystalline components and was well mixed compared with the simple mixture. After the heat treatment, LaNiO3 was obtained from both precursors through three steps of endothermic solid-state reactions. The mechanochemical treatment suppressed the formation of relatively stable hexagonal lanthanum oxycarbonate (La2O2CO3) phase as the intermediate, resulting in the formation of high crystallinity LaNiO3 at relatively low temperature.