This paper describes a micro-macro computational strategy for the analysis of structures which are described up to the "micro" level such as composite structures. This approach splits the solution in a macroscopic part and an additive micro complement. Moreover the resulting algorithm is related to a 2-level "mixed" domain decomposition method, involving an homogenization procedure in order to define the macro-scale problem. It produces an efficient algorithm for solving heterogeneous large-size finite element problems.