A board-to-board electrical connector comprising a male connector including a first insulating base, a plurality of male terminals, each said male terminal having a male contact and a male stator; a female connector including a second insulating base, a plurality of female terminals, each female terminal having a female lead and a female contact extending from the female lead in an bending direction for engagement with said male contact, and a female stator which engages with said male stator, the male contact is a closed ring in shape with at least a protrusion formed at its outside surface, which is restrained in the receiving cavity formed between the first convex of the female contact and the second convex set abreast to the first convex, and the outside surface of which contacts with the first convex and the second convex respectively, the first convex and the second convex being formed with the arm of the female contact bended forth and then back, the protrusion engaging with the first convex or the second convex in a locking manner. The electrical connector disclosed is small in size and the connection of the male terminal and female terminal is reliable.