This research aims at building an instrument to evaluate the linguistic-discursive quality of disciplinary texts produced by law students. A rubric for evaluating writing across the law curriculum was developed using a qualitative study with a phenomenological design. The interviews with teachers used to elaborate the instrument allowed to collect the linguistic-discursive properties that specialized texts should have and the criteria that disciplinary teachers should evaluate in such texts. Once validated through different procedures, the rubric was composed by sixteen dimensions and four macrodimensions, i.e., communicative situation, communication of legal knowledge, global and sentence structure and regulations. The final version of the instrument obtained a considerable agreement in both the content validity (K=0.73) and the intercoder reliability (K=0,62). This validated instrument would be of great value for the different actors involved in the process of forming lawyers for at least two reasons: on the one hand, more precise information regarding the acquisition of the legal competence across the law curriculum could be obtained; on the other, intervention strategies adapted to the needs of the students and tools for developing a didactic of writing in law can be elaborated.