Abstract: The ordered subsets EM algorithm has excited considerable interest for image reconstruction from projection due to its acceleration of the standard EM algorithm.In this paper,we introduce an update algorithm called Symmetric Ordered Subset Expectation Maximization Algorithm(S-OSEM)which accelerates the OSEM as well as satisfies subset balance and orders the subset effectively by the Symmetric structure of projection lines. Furthermore,standard EM also can be accelerated by the Symmetric structure of projection lines.The results indicate that with the appropriate number of subsets,the S-OSEM algorithm can improve the reconstruction speed and imaging precision effectively.