Although it’s considered as a branch of Sciences of Hadith, Science of Defective Hadith is one of the most difficult areas to specialize on. Indeed very few people in this area could utter words. Among these was Imam Tirmidhi who wrote important works especially in the period of emergence and institutionalization of science of Defective Hadith, namely in the third Hijra century which was the mostbrillant period of narrating hadith. Science of Defective Hadith is aknowledge earned through a learning process that bases a long termteacher-student relationship. Tirmidhi has learned that science with theway silsila (sequence) from important personages from this area. Amongthese from whom he benefited mostly on this science was Buhari whowas the authority of ilnesses of the hadith like other hadith sciences, asthe phrase goes he became Buhari’s caliph.Tirmidhi has two works belong to this area: one of them is namedIlalu’s-Sağîr and the other is known Ilalu’l-Kabir or Ilalu’l-Mufred.Ilalu's-Sagir, although it’s placed at the end of the book named el-Camiand it’s minör from the aspect of volume, when the time in which it’swritten and it contains knowledges relating to all of the hadith sciencemore than defective hadith is considered, is very important. Ilalu'l-Kebiris completely related to the ilnesses of the hadith. In this work, Tirmidhidealt with defects of hadith and a lot of other issues. This work is veryimportant because it contains reach information about ilnesses ofhadith and in particular from the aspect of transferring to the nextgeneration the knowledge he learned from his teacher Buhari. In thisstudy, we will try to investigate Tirmidhi and “Ilal-ul Kebir”, the workhe compiled in this field, from the point of the science of Ilal.