A novel optimisation method for designing new and highly complex mode converters is presented. Owing to the complex relationship between the structural parameters and the output performance merit factor, it is very difficult to obtain a general algorithm. As a case study, a mode converter with eight structural parameters is designed with the proposed method. Owing to a large number of structural parameters and complexity of the designing process, the problem is formulated and optimised by using a recent optimisation algorithm called Grey Wolf Optimiser. Six optimal mode converter designs are obtained for LP 01-LP 0m (m = 2, 3,…, 7). These optimal designs outperform existing state-of-the-art designs. Furthermore, achieving these results is done without any human involvement.