In my talk I will address the issue of mass effects on power correction to event shape distributions. In particular I will define an operator that describes the effect of hadron masses on the leading 1/Q power correction. Here, Q is the scale of the hard collision. Despite the fact that different event shapes have different hadron mass dependence, we provide a simple method to identify universality classes of event shapes whose power corrections depend on a common nonperturbative parameter. We compute the anomalous dimension of the transverse velocity operator, which causes the power correction to exhibit non-trivial dependence on Q.