The aim of this study was to evaluate the effect of calcium hydroxide on the fracture resistance of human root dentin at different time intervals. After root canal preparation, one hundred single-rooted premolar human teeth were randomly divided into two equal groups. After placement of calcium hydroxide paste within the canal, one group of teeth was divided into 5 subgroups and incubated for a period of 1 week or 1, 3, 6, or 12 months at 37°C and 100% humidity. The second group of teeth was also divided into 5 subgroups and incubated under the same conditions without placement of calcium hydroxide paste. After mounting the teeth in a Zwick test machine, the force required to break each tooth was determined. Data analysis was performed using the one- and two-way analysis of variance tests. The results demonstrated that the mean force needed to cause fracture differed significantly between the two groups during the first, third and sixth months of incubation (P= 0.001, P< 0.001 and P= 0.035, respectively), and the amount of force necessary for fracture was greater in the control group. Therefore, it would appear from this study that using calcium hydroxide as a long-term intracanal dressing showed a significant decrease in peak load at fracture when compared with the control groups at the end of the first, third and sixth months of treatment and would suggest that using Ca(OH)2 for periods longer than 1 month should be used with caution.