This research work describes the time-delays effect on speed control of DC motor system with non-linear load disturbance in the states. With this task, the paper presents a linearised model of speed control system based on delay-dependent state equation. Since the non-linearly load disturbance in the DC motor system (with time delay) ultimately affects the evolution of the system states, the non-linear load disturbance is modelled (mathematical) as a non-bounded non-linear function of current state vector and delayed state vector, subsequently included into the system stability analyses. In the analysis, two cases are employed: constant time delay and time-varying delay with exogenous load disturbance. In the proposed results, for broad range of PI controller gains, the maximum value of upper bound delay of DC motor speed control system are determined and new stability criterion was identified.Reference to this paper should be made as follows: Venkatachalam, V., Prabhakaran, D., Thirumarimurugan, M. and Ramakrishnan, K. (2019) 'LMI based stability analysis of non-linearly perturbed DC motor speed control system with constant and time-varying delays', Int. ). His research interests include convex optimisation techniques, robust control analysis/synthesis applied to time-delay systems, optimal control and LMI optimisation. This paper is a revised and expanded version of a paper entitled 'LMI based stabılıty analysıs of non-lınearly perturbed dc motor speed control system wıth constant and tıme-varyıng delays' presented at