An electrical connector assembly, comprises a base (4) defining an upper surface and a lower surface, a central processing unit (CPU) (3) assembled upon the upper surface of the base, a printed circuit board (PCB) (5) arranged at the lower surface of the base. A retainer (1) comprises a top cover (11), a bottom cover (12) and a side wall (13) laterally connecting the top cover and the bottom cover, the top cover forms a pivoting structure (111) extending along a first direction at a free edge thereof. A pressing member (2) comprises a shaft (21) being pivotally mounted to the pivoting structure for the pressing plate (23) to press against an upper face of the CPU.