The performance of hybrid organic-inorganic perovskites (HOIP) films greatly depends on their quality. As HOIP films are usually prepared from solutions of its precursors, it is very important to study the evolution process from precursors to HOIPs. But the investigation on the growth of perovskite thin films from precursor solution is limited by their polycrystallinity. Study on single crystals can help gain a better understanding. In this work, methylammonium lead triiodide (MAPbI3) single crystals grown from precursor solutions with different methylammonium iodide (MAI): lead iodide (PbI2) ratios were investigated. We observed a V-shaped dependence of the onset temperature of MAPbI3 crystallization on the MAI: PbI2 ratio. When the MAI: PbI2 ratio is less than 1.7, the crystallization onset temperature decreases with the increasing MAI: PbI2 ratio. However, it increases with the increasing MAI: PbI2 ratio at higher MAI: PbI2 ratio. These are attributed to the effects of MAI on the supersaturation concentration of precursors and the interfacial energy of the crystal growth. At low MAI: PbI2 ratio, more MAI can lead to the supersaturation of the precursors at a lower temperature. At high MAI: PbI2 ratio, the crystal growing plans change from (100)-plane-dominated to (001)-plane-dominated. The latter has higher interfacial energy than the former. Hence, the interfacial energy between the crystal growing plans and the precursor solution increases with the increasing MAI solution, leading to higher crystallization onset temperature.