A semiconductor package and a substrate structure thereof are provided. A solder mask layer applied on the substrate structure is formed with outwardly extended openings corresponding to corner portions of a chip mounting area of the substrate structure. When a flip-chip semiconductor chip is mounted on the chip mounting area and an underfilling process is performed, an underfill material can fill a gap between the flip-chip semiconductor chip and the substrate structure, and effectively fill the outwardly extended openings of the solder mask layer corresponding to the corner portions of the chip mounting area so as to provide sufficient protection for corners of the flip-chip semiconductor chip and prevent delamination at the corners of the flip-chip semiconductor chip during a subsequent thermal cycle.