Background: The use of short message service (SMS) is very popular among youths. Recent research has shown that it has some adverse consequences such as dependency. Objective: The present research aimed to determine the frequency of SMS dependency and its association with the temperament and character of medical students. Methods: A total of 293 medical students participated in this cross-sectional study in 2015 in Mashhad, Iran. The study group was divided into four groups according to their grade (using stratified sampling). Temperament and character inventory (TCI) and an SMS addiction test (SAT) were distributed between participants. SPSS software (version 11.5) was used for data analysis. Linear regression, Spearman's Rank-Order Correlation, Independentsamples t-test, ANOVA, Mann-Whitney U, and Kruskal Wallis were used for data analysis. P-value less than 0.05 was considered significant. Result: The mean age of participants was 24.44±4.63. Novelty seeking (p=0.003), cooperativeness (p<0.001), self-directedness (p<0.001) and self-transcendence (p=0.003) showed significant inverse correlation with total SAT score. There was significant difference between the mean of the total SAT in three state variables of novelty seeking (p=0.02), cooperativeness (p<0.001), self-directedness (p<0.001) and self-transcendence (p=0.02). The residents group had the highest mean SAT score. There was a significant difference in the total SAT scores according to the grade of the student (p<0.001). Moreover, we compared the participants' temperament and character with Iranian normative data (derived from a study conducted in 2005) and found that the mean score in each area differed significantly between the two datasets (p≤0.001). Conclusion: Use of communication technologies is different according to temperament. Considering the key role of medical students, more attention needs to be paid to cellphone usage in this group.