The purpose of the present study is to determine the tensile properties of the single collagen fibrils with various diameters using our original tensile test method. The fibrils were directly isolated from mouse tail tendons. The both ends of the fibril were wound onto the tips of microneedles several times using micromanipulators. The fibril and tips were immersed in physiological saline solution. Then, the fibril was stretched to failure by moving the microneedle. The tensile properties of the 26 fibrils with various diameters ranging from 140 to 490 nm were determined. The stress-strain curves of these fibrils were almost linear or upward convex at the large strain region. These fibrils showed the tensile strength of 47-310 MPa, the strain at failure of 7.3-81 %, and the tangent modulus of 160-1600 MPa. The tensile strength and tangent modulus decreased significantly with increasing the diameter. On the other hand, the strain at failure increased significantly with increasing the diameter.