The aim of the present study was to test the hypothesis that the fracture strength of calcium hydroxide and mineral trioxide aggregate (MTA) filled immature teeth decrease over time. Immature mandibular incisors from sheep were extracted and the pulps were extirpated using an apical approach with a barbed broach, and the teeth were divided into three experimental groups. Group 1: Untreated teeth. Group 2: The root canal was filled with calcium hydroxide paste (Ultradent-UltraCal XS). Group 3: The root canal was filled with ProRoot MTA system (Dentsply, USA). All specimens were kept in saline with 1% antibiotics at 4 degres C for certain periods of time: two weeks, two months and one year. All teeth were tested for fracture strength in an Instron testing machine at the indicated observation periods. The results were subjected to statistical analysis by a one-way analysis of variance and with the t-test at a 5% level of significance. One tooth from each group was selected randomly for the histological study. The mean fracture strengths decreased over time for all the three groups. The fracture strengths were not found significantly different from the untreated, calcium hydroxide-treated or MTA-treated teeth at two-week or two-month (p>0.05). However, the results for MTA-treated teeth were significantly higher than the other two groups at one year (p=0.0137). The teeth with root treatment with MTA showed the highest fracture resistance at one year (p<0.05), since only MTA induced the expression of TIMP-2 in the dentin matrix and possibly prevents rapid destruction of collagen.