A shareholder derivative suit allows shareholders to file a suit against directors on behalf of the company when the company refuses to sue such directors for wrongful acts and negligence against the company. The purpose of a derivative suit is to mediate damages resulting from the companyûs failure to seek redress against such directors. After bringing a derivative suit to a court, the legal proceedings thenceforth is in pursuant to the procedural rules which also apply to other cases. Therefore, this article focuses mainly on whether the Thai Civil Procedure Code is proper for a shareholder derivative suit, in particular on whether shareholders may join the cases which are already filed or will be filed by other groups of shareholders. A study shows that the civil procedure code allows other shareholders to become co-plaintiffs in a derivative suit either before or after filing the case by filing an interpleader. However, this rule is not proper for a shareholder derivative suit because it differs from other types of civil cases. This article suggests that the civil procedure rules regarding co-plaintiff should be amended and further suggests that details and conditions regarding co-plaintiff in a shareholder derivative suit should also be specified in the amended rules in order to render congruency with the principles behind such a suit. Furthermore, the current civil procedure rules is incongruent with shareholders rights in that it forbids shareholders to commence a new proceeding on the same grounds where a judgment or order has become final. This article suggests that such a rule should be also amended in order for a derivative suits to achieve its main purpose.