An incline angle adjustable stair climbing machine includes a base frame, a tilting seat pivotally mounted at the base frame, a tilting seat adjuster for adjusting an incline angle of the tilting seat relative to the base frame, a staircase mounted at the tilting seat and including a plurality of tread boards biasable relative to the tilting seat, and a tread board adjuster mounted at the tilting seat for adjusting a biasing angle of the tilting seat to maintain the plurality of tread boards constantly horizontal for enabling a user to conduct stair-climbing exercises at different climb gradients while maintaining good posture.