Mental action and reaction of those commiting crimes, known as the scond mental base of the crime, happen in two ways; in some cases, those commiting crimes act against penal laws intentionally while in other cases, they act without enough consideration and, therefore, commit a crime. The former would be regarded as intentional and the latter unintentional. The paper, using descriptive-analytic method, studies the definition and principal qualities of unintentional crimes in a new approach on the one hand, and introduces the ways to prevent them on the other hand. As shown in the study, in preventive studies, regarding the former and the latter both, we should oppose those agents of fundamental part in happening crimes; that is, those given their destruction, no other criminal element would be formed.