The objective of this work is to construct an instrument that allows describing the moral reasoning of adolescents in conflict with the law, through research methodological cut whose purpose is to improve the methods used in other investigations and therefore, develop effective, useful and reliable instruments that can be applied by other people interested in the investigation. The construction process was carried out by means of seven phases that began by the investigation and understanding of the different existing theories about the moral reasoning, as well as its relationship with the other fundamental axes, adolescence and acts criminals; and that culminated in the construction and validation of an instrument (IRCMA). East It has content validity and can be applied to adolescents between 14 and 17 years of age have entered the Criminal Responsibility System for Adolescents. From what found, it is recommended the continuity of the project that includes piloting and application of the instrument, as well as its use for future research so that it can contribute to the understanding of moral reasoning in adolescents who have committed crimes.