In this paper, two kinds of multirate formulation are considered in terms of control performance and computational efficiency. One is a periodic time varying formulation and the other is a time-invariant formulation. They will he compared in terms of a linear quadratic performance index and computational burden. For simplicity of comparison, we consider the periodic formulation of Colaneri and,Nicolao and derive a time-invariant formulation which implements a concept analogous to the selection matrices used in their periodic formulation. A comparison of the two formulations shows different control performances due to the noise added through the evolvement and the measurement of the system, even though the same linens quadratic Gaussian performance index is applied. The computed performance index can be used to select the appropriate multirate formulation or to determine an appropriate combination of multi-sampling rate.