Aims: To evaluate the effect of intracanal cryotherapy on surface temperature change in the root apex with different thickness. Study Design: Comparative Study. Place and Duration of Study: Operative Dentistry Department: Altamash Institute of Dental Medicine, Karachi, Pakistan: July 2020 till June 2021. Materials & Methods: Sixty extracted single-rooted teeth were randomly divided into two groups: 30 each with one group consisting of teeth with ≤3 mm root thickness and another group consisting of teeth with thickness >3mm. Both groups were equally subdivided into Group A and Group B. After endodontic preparation of teeth, Group A underwent room temperature saline irrigation and Group B cold temperature saline (2.5°C) irrigation. A digital thermometer was used to measure the external root surface temperature by placing the thermocouple at 2 mm from the apex of the root’s buccal surface. Results: Initial and final temperatures were compared using paired t-test. Independent t-test was applied to compare the final temperature between ≤3mm and >3mm thickness groups. P-value less than or equal to 5% level of significance was defined as statistically significant. In group A, An average temperature decrease of 4.4 ± 0.5°C was observed in the final temperature when thickness was ≤3mm (p=0.040) while a reduction of 2.6 ± 0.5°C was seen in the final temperature when thickness was >3mm (p=0.008). When thickness was ≤3 mm, final temperature among group A and Group B was significantly different with higher mean final temperature in Group A (p<0.001). At thickness >3mm, final temperature between Group A and Group B was different with significantly lower final temperature in Group B than Group A(p<0.001). Conclusion: The external root surface temperature reduction was found to be inversely related to the external root thickness. Thin roots showed greater temperature reduction compared to thick roots.