In this paper, we propose a new digital watermarking scheme for the three-dimensional (3-D) mesh model in the spatial domain. In order to insert and extract watermark signals, we generate triangle strips by traversing the 3-D mesh model. We embed the watermark signal into vertex positions of the 3-D model in the spherical coordinate representation. We define a new masking operation for the watermark signal according to the variation of vertex positions along the traversed strips so that we can change the embedding strength adaptively and reduce the visibility distortion of the data embedding. We test the proposed watermarking scheme by embedding a random binary sequence and applying different attacks, such as additive random noise, compression by MPEG-4 SNHC, and mesh simplification, which shows the robustness of the watermarking scheme.