A machine with large output stiffness variation can meet the requirement in different working environments. This paper presents an approach to design a linear variable-stiffness mechanism (LVSM) with adjustable zero-to-infinite stiffness. The idea is to use circular slots to adjust the initial rotation of two parallel connected springs. The output force to displacement curve can exhibit zero to very large stiffness depending on the rotation of the springs. Infinite stiffness is achieved by using mechanical stoppers to constrain the displacements of the springs. The merit of the proposed LVSM is that zero and infinite stiffness can be simultaneously achieved in a compact space. To further reduce size and complexity, specifically designed planar springs are proposed to replace commercially available coil springs. Force and stiffness analyses are presented to design a LVSM with the largest stiffness variation. The effects of various parameters on the stiffness variation are discussed. The results are numerically verified with a prototype illustrated.