The problem of service composition is of key importance in the Internet of Things paradigm. A composite Web Service can clearly determine how real life objects described as Web Services could interact with each other. This article proposes a non-functional parameters aggregation algorithm, used in composite service plan optimisation stage, taking into account the uncertainty of the nonfunctional parameters. Also, an optimisation algorithm, using the proposed aggregation method, is presented to show the complete solution for the problem. In this proposition, the algorithm focuses on composite service plan optimisation with uncertain parameters with the given uncertainty description on example of uncertainty parameter. However, the proposed algorithm can be generalized, so it can be used also with other uncertain parameters