For the aim of third harmonic generation (THG) experiment of NdYAG 1319 nm laser to produce 440 nm blue laser using KTP crystal at room temperature, theoretical calculation and experimental study have been carried out for the THG phase-matching angle of KTP crystal. Using several sellmeier equations, phase-matching angles are derived, and corresponding effective nonlinear coefficients are calculated. One result (θ=84.6°, φ=0°) of the angles is chosen to cut KTP crystal, and then by using a laser of 1319 nm, crystals are carried into the cavity, methods of rotating the crystal and adjusting the temperature to find out optimum phase matching angle of the KTP crystal for THG (θ=85.04°, φ=0°) are applied. After re-cutting KTP crystal, the beam intensity of outputting 440 nm blue laser has a significant improvement and the optimum operating temperature is 18 ℃.