The present invention relates to a semiconductor device. The semiconductor device includes a substrate and a chip. The chip is electrically connected to the substrate. The chip includes a chip body, at least one chip pad, a first passivation, an under ball metal layer and at least one metal pillar structure. The chip pad is disposed adjacent to an active surface of the chip body. The first passivation is disposed adjacent to the active surface, and exposes part of the chip pad. The under ball metal layer is disposed adjacent to the chip pad. The metal pillar structure contacts the under ball metal layer to form a first contact surface having a first diameter. The metal pillar structure is electrically connected to a substrate pad of the substrate to form a second contact surface having a second diameter. The ratio of the first diameter to the second diameter is between 0.7 and 1.0. As a result, the first contact surface and the second contact surface have an equivalent bonding force, which prevents the metal pillar structure from cracking due to a shear stress. Thus, the structure strength of the semiconductor device is enhanced and the semiconductor device can pass the reliability test.