Phase-matched type Ⅰ second-harmonic generation in a φ10 mm KTP crystal sphere using a 35 picosecond pulse as pump laser is reported. The type Ⅰ phase-matching (PM)curve of KTP crystal is measured directly according to the SHG conversion efficiency, and the corresponding nonlinear optical coefficient is calculated. By comparison of the experimental PM curve with the theoretical PM curves calculated from six Sellmeier’s equations, it is found that the equation given by Vanherzeele is the best one.