Membrane system is a computing model which imitates natural process at cellular level. In this system all objects can evolve in a maximal parallelism and distributed manner. Recent results show that this model is a promising framework for solving NP-complete problems in polynomial time. The paper proves the possibility to perform operations with integer numbers in a membrane system, and gives an effective method to implement arithmetic operations, which seems to have a lower complexity than when implementing them in usual computer architecture.