On a rear side of seat back, a trim cover assembly of the seat back includes an openable first cover portion disposed centrally thereof and a pair of second cover portions disposed on opposite sides of the first cover portion. A pair of female fastener assemblies are provided to two lateral ends of the first cover portion, respectively, and each comprises: an anchor element; a female fastener element; and a connecting element connected between the anchor and female fastener elements. The anchor element is securely engaged with a seat back frame, thereby neatly securing the two lateral ends of the first cover to the rear side of seat back. The male fastener element is securely engaged with the female fastener element, thereby neatly securing the two second cover portions to the rear side of seat back in alignment with the two lateral ends of the first cover portion.