This paper is concerned with the guaranteed cost control problem for continuous-time periodic piecewise linear systems with time delay. Sufficient delay-dependent conditions of closed-loop asymptotic stability are presented based on an improved formulation, which uses a novel Lyapunov-Krasovskii functional with relaxed requirement in positive definiteness of the involved symmetric matrices. The corresponding optimization problems aiming at the mixed performance involving an upper bound of H-2 guaranteed cost and an H-infinity performance index for disturbance attenuation are established. By designing an iterative algorithm subject to the proposed conditions, the periodic guaranteed cost controller gains over each sub-interval are tractable for the resulting closed-loop time-delay system. The effectiveness and reduced conservatism of our proposed criteria are validated and illustrated via numerical simulations. (C) 2018 Elsevier Ltd. All rights reserved.