Illegality objections in investor-state arbitration raise several issues in jurisprudence. It cantered on the extent to which the state can use the mechanism of illegality objection in domestic courts can justify their failure to accord international protections to an investor by evoking the illegality objection. Similarly, states can use the illegality objection as a defence against claims brought against it by an investor, with the aim of depriving the jurisdiction of an arbitral tribunal due to the illegalities of the contract or its performance. Therefore, the extent to which the illegality objection is a strong defence or whether the state can successfully raise the objection is a matter that requires further research and analysis of the jurisprudential views and judicial principles used by courts in investor-state arbitrations. The paper discusses the concept of illegality objection, the condition of the legality of investments as the basis of the arbitral tribunal's jurisdiction, as well as the procedural and substantive requirements for illegality objections and the legal effects resulting from accepting illegality objections, through discussing and analysing the jurisprudential issues of the arbitral tribunal's rulings.