Network-on-Chip (NoC) is a newly introduced paradigm to overcome the communication problems of System-on-Chip architectures. Mapping applications onto mesh-based NoC architecture is an NP-hard problem and several heuristic methods have been presented to solve it so far. Scalability is the main problem of the heuristic methods and it is very difficult to conclude that one heuristic is better than the others. Integer Linear Programming (ILP) based methods determine the optimum mappings. However, they take very long execution times. In this paper, we propose a clustering based relaxation for ILP formulations. Our experiments conducted on several multimedia benchmarks and custom graphs show that the proposed method obtains optimal or close to optimal results within tolerable time limits. (C) 2011 Elsevier Ltd. All rights reserved.