A battery pack including: a bare cell; a holder disposed at an end of the bare cell, having a connection groove on a side surface thereof; a protection circuit board seated in the holder; and an outer case disposed upon a side of the bare cell, having a connection protrusion mated with the connection groove, to secure the holder. Another battery pack includes: a bare cell including an electrode assembly and an cell case to accommodate the electrode assembly, including an outer casing and a inner casing, the cell case having a connection protrusion disposed at an edge of at least one of the outer and inner casings; a holder disposed at an end of the bare cell, having a connection groove to mate with the connection protrusion and thereby secure the holder; and a protection circuit board seated in the holder.