In this study, the prohibition of participation in negotiations of the members of the board of directors of joint stock company will be analysed. Firstly, in the event that the member of the board of directors is a legal person, the question of whether the prohibition shall be applied in cases where the interests of the real person attending the meetings on behalf of the legal person (Art. 359.2 TCC) and his/her relatives listed under Art. 393.1 TCC conflict with the interests of the company. Subsequently, it will be evaluated whether the application of the prohibition in cases where the failure of the board member to participate in the negotiations is required by the rule of good faith is of a nature to include the personal interest within the company as well as the personal interest outside the company. It will also be clarified whether the member can be present at the meeting while the matter constituting a conflict of interest is being discussed and whether the prohibition covers voting. The other focal points of our study will be whether the decision taken in case of violation of the prohibition will be valid or not, and if this question is answered in the negative, firstly, the type of invalidity, and secondly, whether the invalidity of the decision to be taken on the matter constituting a conflict of interest in the presence of prohibited members should be concluded in terms of both meeting and decision quorum or only in the context of decision quorum. Finally, the invalidity of the transaction based on the board of directors’ decision taken in case of violation of the prohibition will be analysed. While addressing the aforementioned issues, de lege ferenda suggestions will also be made regarding Art. 393 TCC.