The signs of nonlinear optical (NLO) coefficients of La2CaB10O19 (LCB) crystal were determined by Maker fringe method and phase-matching method. Compared with d22, the signs of d21 and d23 are found by Maker fringe method to be negative and positive, respectively, and the sign of d14 is found to be positive by phase-matching method. Phase-matching angles for third harmonic generation (THG) in LCB crystal were calculated and verified by experiments. The distribution of effective NLO coefficient (deff) and the optimal phase-matching angles for THG with LCB crystal are determined for the first time, with optimal phase-matching angles of (48.7°, 72.5°) and (65.3°, 37.5°) for type-I and type-II THG, respectively. The results show that LCB is an excellent NLO crystal for THG application.