A foldable mechanism for treadmills includes two respective foldable sets and an operation bar is connected to the two foldable sets. Each foldable set has an operation member fixed to the upright frame corresponding thereto and the operation member has a groove defined in a side thereof. A periphery defining the groove includes multiple recesses. Two end plates each have a first end fixed to one of two ends of the operation bar and the other end of each end plate is pivotably connected to the foldable set corresponding thereto. An engaging member extends from the first end of each end plate and is movably inserted into the groove of the operation member. When pivoting the upright frames toward the horizontal frame of the treadmill by pivoting the operation bar, the engaging members are engaged with one of the recesses in different folded positions.
