A muscle stretching structure for a foot massager includes a massaging base, two rocking boards, a first supporting element and a second supporting element. The massaging base has two bottom spaces, a first supporting surface and a second supporting surface, and a lifting unit disposed on each first supporting surface. Furthermore, a bottom side of each rocking board is attached to a respective rocking module. The first supporting element has two containment spaces and is pivoted to the first supporting surface of the massaging base with the first hinges. A second supporting element has two upper spaced and a driving unit is attached to the bottom of the second supporting element. The driving unit is disposed between the second supporting element and the second hinges. The rocking board can be rocked by the rocking module to perform a stretching movement upon the feet, and the first supporting element and the second supporting element are driven by the lifting unit and the driving unit to rotate in a direction opposite to the rocking direction of the rocking board so that the user's leg muscles are treated to both massaging and stretching effects.