The present invention relates to a monofilament suture and a preparation method thereof, more specifically, to a novel monofilament suture wherein two polymers having identical inherent properties are spun to have a cross-sectional structure of conjugated filaments, and said cross-section has interfaces between the spun polymers. The interfaces form a discontinuous cross-section along the radial direction and are distributed continuously along the fiber axis direction, thereby offering improved knot security and applicability to a variety of medical materials, and a preparation method thereof.