This paper presents a novel approach to design a sliding mode control using a new computationally efficient formulation of Multirate Output Feedback (MROF), that applies to models in a special form in which some of the states of the system are directly available as outputs. The computation of the remaining states of the system requires matrices of lower dimension and thus the method is less susceptible to ill-conditioning. The proposed method has been successfully applied to the spatial control problem of a large Pressurized Heavy Water Reactor (PHWR), in which the outputs are the 14 zonal power levels. Also, the zonal power levels are selected as state variables along with other variables. Thereby, the PHWR model is perfectly suitable for the application of this new formulation. The non-linear model of PHWR including xenon and iodine dynamics is characterized by 70 state variables and 14 inputs and outputs each. Linear nodal model is obtained by linearizing the non-linear dynamic equations of the reactor about the full power operating point. The non-linear simulation results in representative transients produced by the proposed method are found to be superior to other methods. (C) 2009 Elsevier Ltd. All rights reserved.