We have investigated the critical behaviour of a La_0.8Sr_0.2MnO₃ polycrystalline sample near its ferromagnetic - paramagnetic phase transition temperature by determining the critical exponents corresponding to various thermodynamical quantities. Such a study is very important and interesting because this magnetic phase transition is associated with a simultaneous electronic phase transition, i.e. a metal-insulator transition. In this paper, we report our results on this magnetic phase transition analysed through various techniques like the modified Arrott plots, Kouvel-Fisher method, scaling-equation-of-state analysis and the critical magnetisation isotherm. The values deduced for the critical exponents are in good agreement with those predicted, theoretically, for the mean field theory, indicating that the exchange interaction is of long-range type.