At the present, there is the Civil Procedure Code Amendment Act no. 26 (B.E. 2558) on class action, in Thailand. which has resolved problems in the case of the number of parties having the same fact and point of law in order to reduce the quantity of the cases raised into the court and to save cost and time for proceedings. In addition, the judgment direction has been occurred. However, such law has specified only the plaintiff class action. There is not defendant class action. On the basis of the results of this research, it found that, in case, the number of infringing or default parties still have had to separate their counterclaims into each case as well as the Civil Procedure Code currently applicable is not sufficient to solve the said problem. This has led the quantity of case increasing and the waste of time and cost for such proceedings. This thesis emphasizes to study the concept and style of defendant class action shown in overseas: USA and Canada in comparative with the significant conditions of Thai law regarding the class action and to analyze and compare issues of entering into the class action, qualifications of the group’s representative, the ability of members of a defendant class to opt out from the class, and determining the attorney’s fee on defendant side. The researcher therefore would recommend a legal measure on defendant class action as another channel which may revise some subject matter of the Civil Procedure Code Amendment Act no. 26, totaling 5 issues: (1) Determining the nature of such group of persons, it should be revised its definition to be defendant class action by changing from the definition that more members of a class may sue or be sued who have the same right as the result of “The same of fact and law”. For the number of persons, the existing principle of the Civil Procedure Code Amendment Act no. 26 should be applied with the concept of appropriateness of the number of groups of Federal Rules of Civil Procedure Rule 23 of US; (2) Criteria on determination of the qualifications of the representative or the attorney of the defendant should be drawn clearer; (3) For the issue of opt out, normally, the court have the power to use discretion about right of opt out of member ; (4) Cost of class action; and (5) Determining the defendant attorney’s reward, the court shall have a role to determine and consider the attorney’s reward as appropriate and there should arrange a government fund to support the cost and attorney’s fee for the proceedings.