A washer includes a wedge-shaped main body, a wedge-shaped adjusting portion moveably positioned on the main body and a flexible connecting portion interconnecting the main body and the adjusting portion. The main body defines a through hole through which the screw passes. The main body includes a bottom surface and a positioning surface angled relative to the bottom surface. The adjusting portion includes a flat contact surface and a latching surface contacting the positioning surface of the main body. The latching surface is angled relative to the contact surface. A wedge angle of the adjusting portion equals that of the main body, thus the bottom surface of the main body is substantially parallel with the contact surface of the adjusting portion.