The initial growth processes on InP 11̄00 and InP 112̄0 surfaces are theoretically investigated to clarify temperature and pressure dependence of InP nanowire shape fabricated by selective area growth. Our Monte Carlo simulations in conjunction with density functional calculations for the growth processes with a realistic environment such as temperature and pressure reveal that there are several growth modes depending on the growth conditions. Due to lower growth rate on InP 112̄0 surface compared with InP 11̄00 surface, the nanowires grow along both vertical  and lateral <112̄0> directions for low temperatures with high V/III ratio conditions. On the other hand, at high temperatures with low V/III ratio conditions, the nanowires grow along only vertical  direction and form 11̄00 side facets due to lower surface energy on InP 11̄00 surface. The adsorption of P atoms on the surfaces are found to be crucial for the growth along the lateral direction of nanowires. The obtained results offer a possibility to predict and control nanowire shape by tuning growth conditions such as temperature and pressure.