Every criminal justice system, including the jurisprudential criminal system with direct impact on Iran's criminal law, seeks to identify, use and organize the most effective and fair" legitimate" tools to discover the truth in order to provide the possibility of achieving justice and issuing just verdicts by forcing judges and other criminal justice activists to use these tools. Using the phrase "discovery of truth" for twelve times in the Criminal Procedure Code shows that in contrast to some interpretations of jurisprudential rules, the legislator, considers the purpose of the trial not the disposal of hostility but the discovery of truth. However, there are some jurisprudential rules and sub-rules about demonstrative evidence, which accepting them unequivocally can make the path difficult to reach the goal of truth discovery. The legislator has tried to overcome the self-made obstacles to the discovery of truth by moving from the devotional approach to the rational approach, from the quantitative approach to the qualitative approach, from personal knowledge to common knowledge, and from the subjectivity of evidence to the methodology of them. This article seeks to study the approach of the present positive law to the acceptance of new and scientific demonstrative evidence without compromising the relevant jurisprudential structures, under each of these departures and with emphasis on Qasameh. And in order reach our goal, we also used descriptive-analytical method and described those sub-rules governing the demonstrative evidence in Imami jurisprudence that dogmatic and inflexible belief in which can cause obstacles in the way of discovery of the truth.