The model field coupling theory was applied to calculate the coupling efficiency of a Gaussian laser beam to an optical fiber via a single lens. The dependences of the coupling efficiency on the focal length of the lens were investigated for both single-mode and multi-mode fibers. An experiment was conducted to couple a 532 nm laser beam with a M2 factor of 1.05 to both a single-mode fiber with 3.5 μm-core and a multi-mode fiber with 25 μm-core. Good agreement is obtained between the experimental results and the theoretical calculation.