In this note, the problem of designing sliding-mode controllers for uncertain systems is considered. Based on the LMI approach we develop a new design method of linear sliding surfaces which are linear to the state. We give existence conditions of linear sliding surfaces in terms of LMIs and we give characterizations of linear sliding surfaces, thus one can easily design sliding-mode controllers for even large-scale systems by solving given LMIs numerically very efficiently with various powerful algorithms. Finally, we consider some issues relating to the advantages of the proposed LMI-based formulation.