The effect of trace element P on the microstructure and mechanical properties at high temperature for stoichiometric NiAl as extruded were investigated. The results showed that the little doping P mainly segregated at the grain boundaries and influenced the lattice parameter of NiAl and the elongation at high temperature. The segregation of P at the grain boundaries impeded the process of dynamic recovery or recrystallization during deformation at high temperature, which aggravated to the formation of cavities at the grain boundaries and this made the different mechanical properties between pure stoichiometric NiAl with and without phosphorus. The difference mainly revealed as follow. （1） the alloy doping little P experienced long time hardening in the true-stress vs true strain curves; （2） the largest elongation was decreasing during the deforming process; （3） the deformation activation energy and the strain rate sensitivity index were increasing and decreasing, respectively. The deforming mechanism was the comprehensive role between dislocation gliding and climbing behavior at high temperature.