A three-dimensional (3-D) analytical indoor line-of-sight (LOS) probability model is given in this letter. According to the indoor layout of a specific building, the proposed closed-form model can be used to compute the LOS probability in a 3-D environment. The model is validated in both single rooms and a typical office building by Monte Carlo simulations. Moreover, the coverage rate in a typical office building with a 3-D ultra-dense small cells network is evaluated based on the proposed LOS probability model. The numerical results show that the proposed model matches the simulations well in terms of both the LOS probability and the coverage rate.