This paper presents both an error modeling of an odometry system and a possible procedure in order to evaluate this error. This error contains systematic and non-systematic components. In this paper both components have been evaluated for the mobile robot Nomad150. The non-systematic errors are expressed in terms of a covariance matrix which depends on the path followed by the mobile robot. In contrast to previous approaches which require to assume a particular path (straight or circular) in order to compute this covariance matrix, here general formulas are derived. In these formulas there are some parameters which depend on the robot and on the environment where the robot moves. An experimental procedure, only based on the odometry data (i.e. which does not require any other sensor to know the actual robot position) is also given, by which it is possible to estimate the parameters intervening in the formulas derived.