The Islamic Republic of Iran is in terms of the number of criminal population، in the eighth and in terms of the number of pre-trial detentions in the ninth place in the world. An increase in the rate of prison population from 172 in 1993 to 287 in 2014 and an increase in the rate of pre-trial detention from 39 to 72 in the same period are clearly showing that Iran is experiencing a huge increase in the criminal population. The growth of the rate of pre-trial detention is one of the main causes of penal population growth and the subsequent overcrowding in prisons in Iran. Increase in the rate of pre-trial detention، in turn، results from the legislative، judicial and several other factors. Expansion of substantial criminal law interventions-through the widespread criminalisation and imprisonment، war on drugs and the lack of adequate criminal measures to replace non-custodial measures and eventually legislature's weakness in regulations relating to the custody are the most important factors including legislative and excessive reliance on and inefficient judges and prosecutors to control crime، and minimal use of non-custodial measures and structural weaknesses in the judiciary، are also some other causes of the increase in the number of the defendants waiting for trail.