Collagen serves as an essential structural material in the human body. Despite the complex mechanical conditions surrounding the collagen hydrogels, previous studies mostly focus on analyzing the mechanical behavior under dynamically varying compressive or shear loads, but the tensile properties at the quasistatic time scale are relatively less studied. This work aims to investigate the quasistatic tensile behavior of reconstituted collagen hydrogels under uniaxial tensile stresses. The evolution of the collagen fiber network structures with straining is visually observed using the confocal microscope equipped with the tensile strain actuator. While the unfolding of the initially undulated fibers accommodates the early-stage strains, the deformation mechanism continuously changes to the stretching of fibers through the network alignment to the tensile direction. This transition commences with the buckling of a fiber lying transverse to the loading direction, which otherwise locks the rotation of adjacent fibers.