As well, it is consider the possibility seeking trade off solution between this two criterions. With regular forming models of maintenance is possible to make optimisation, regarding, to use the best maintenance system. If the all important requests and limits are preciosly given then it is possible to solve this kind of problem. The base of presented metodology is presenting with reliability parameters of analysed systems given from system behaviour folowing, from aspect of failure happen, in real conditions of exploatation and costs of theirs maintenance.