The influences of boron addition on secondary phase α-Fe in LaFe11.5Si1.5 melt-spun ribbons were investigated by SEM, TEM and XRD. The magnetic transition and magnetocaloric effect of LaFe11.5Si1.5Bx compound (x = 0.5, 0.7, and 1.0) were discussed. Compared with LaFe11.5Si1.5, the melt-spun ribbons have much finer microstructure after B addition, and even amorphous structure is obtained when B content is higher. The melt-spun specimens were crystallized into La(Fe,Si)13 compound structure of cubic NaZn13-type after annealing at 1000 °C for 2-10 h. The annealing time can be reduced after B atom introduction to get single phase structure and the amount of residual α-Fe is decreased significantly. Magnetic property test show that Curie temperature of the LaFe11.5Si1.5Bx compound (x = 0.5, 0.7, and 1.0) increased slightly with B content increasing; the LaFe11.5Si1.5B0.5, i.e. low-B-content compound, exhibited first-order magnetic transition characteristics and had the same giant magnetocaloric effect with LaFe11.5Si1.5.