A belt tensioner to be applied to a bike, the bike comprising a frame and a belt, the frame comprising a chain stay and a seat stay, the chain stays and the seat stay respectively having a top surface and a bottom surface, the belt tensioner comprising: a tensioner skeleton pivotally connected to the chain stay wherein an included angle is formed between the tensioner skeleton and the chain stay; a roller pivotally connected to the tensioner skeleton and being in contact with the belt; and a tension regulator for pressing against the belt so as to adjust a tension of the belt, wherein the tension regulator is disposed at an end of the tensioner skeleton, and the end of the tensioner skeleton is positioned distal to the bottom surface, wherein a part of the belt is in contact with the tension regulator, and wherein a vertical height the contact part of the belt greater than a height of the top surface, when the tension regulator presses against the belt; wherein the tensioner skeleton further comprises a first link and a second link, the first link being disposed at the chain stay and having a first extension direction, and the second link comprising a first end pivotally connected to the first link, wherein the second link has a second extension direction when the first end is pivotally connected to the first link, and the included angle is formed between the first extension direction and the second extension direction; wherein the second link comprises a second end, and the second end is pivotally connected to the roller; characterized in that the tension regulator further comprises a positioning holder and an elevating stud, the positioning holder being connected to the tensioner skeleton, and the elevating stud being movable relative to the positioning holder and being able to increase the tension of the belt by moving upward to press against the belt. a tensioner skeleton pivotally connected to the chain stay wherein an included angle is formed between the tensioner skeleton and the chain stay; a roller pivotally connected to the tensioner skeleton and being in contact with the belt; and a tension regulator for pressing against the belt so as to adjust a tension of the belt, wherein the tension regulator is disposed at an end of the tensioner skeleton, and the end of the tensioner skeleton is positioned distal to the bottom surface, wherein a part of the belt is in contact with the tension regulator, and wherein a vertical height the contact part of the belt greater than a height of the top surface, when the tension regulator presses against the belt; wherein the tensioner skeleton further comprises a first link and a second link, the first link being disposed at the chain stay and having a first extension direction, and the second link comprising a first end pivotally connected to the first link, wherein the second link has a second extension direction when the first end is pivotally connected to the first link, and the included angle is formed between the first extension direction and the second extension direction; wherein the second link comprises a second end, and the second end is pivotally connected to the roller; characterized in that the tension regulator further comprises a positioning holder and an elevating stud, the positioning holder being connected to the tensioner skeleton, and the elevating stud being movable relative to the positioning holder and being able to increase the tension of the belt by moving upward to press against the belt.