This paper presents an efficient algorithm for encoding the connectivity information of triangular meshes. In the previous algorithms, Huffman or arithmetic coding method is directly used to encode operator series, but in comparison in this method, it can efficiently improve the compression ratio of connectivity information by predicting correctly the operator currently being encoded. By the method, all triangles are traversed first to obtain operator series. Then an arithmetic coder based on variable code-mode is applied to encode the operator series. According to the operator last encoded, the property of triangular mesh and the method of mesh traversal, a code-mode is calculated for each operator currently being encoded, where the operator with higher prediction probability is given a shorter binary strand. Then the binary strand can be obtained according to its code-mode and encode every bit of this binary strand by adaptive arithmetic coding method. The algorithm is a face-based method and also a single-resolution lossless compression method for manifold triangular mesh. Testing results show that the compression ratio of the algorithm is very high and even higher than the compression ratio by using TG algorithm, which is commonly regarded as one of the best in terms of compression ratio.