Abstract: A new semicontinuum model is presented to analyze the dynamic characteristics of silicon nanobeam in this paper. Compared with tradition continuum theory, the new model utilizes the Keating potential and accounts for the discrete nature in the thickness and width directions of the nanobeam. Referring to Sun-Zhang model and using energy conservation theory, the modified Keating model is built. Then the fundamental frequencies of nanoscale clamped-clamped silicon beam are calculated, and the surface effect is considered into the calculation. It is shown that the results based on the new model coincide well either with the results using Material StudioTM software in nano-size or with those based on the continuum model in micro-size. Meanwhile, the model can reflect the fundamental frequency changing with the width, it also accords with the actual experiment.