This paper has two contributions. One is introduction of loop phase margins for multivariable control systems, which extends the concept of phase margin in SISO systems to MIMO systems. The other one is development of an algorithm for computing the loop phase margins. The algorithm is composed of two steps. The first is to find the stabilizing ranges of loop time delay perturbations of the M I MO system using an LMI-based stability criterion derived here. The second is to convert these stabilizing ranges of loop time delays into the stabilizing ranges of loop phases. Two numerical examples are given to illustrate the effectiveness of the proposed approach. (c) 2007 Elsevier Ltd. All rights reserved.