The paper presents the result of a research based on the concept “didactical contract” which  was  introduced  by  G.  Brousseau  in  1980.  On  the  basis  of  this  concept,  we predicted students’ errors and verified by the investigation of 60 students who are studying in the “Long Phú” high school – Tam Binh District, Vinh Long Province. These results indicated that the rules of didactical contract could be reasons for students’ errors in solving mathematical problems.