In order to determine the effect of temperature on germination response of fennel seeds, in Petri-dishes, seeds of 3 varieties (Hamedan, Malayer and Isfahan) were exposed to 6 constant temperatures (10, 15, 20, 25, 30 and 35°C). Results indicated that the temperature had a significant effect on the maximum germination percentage, germination uniformity, germination rate, and time to 5, 10, 50, 90 and 95% germination, while insignificant effect of variety and interacted effect of temperature and variety was observed. Among three non-linear regression models used in assessment (segmented, dent-like and beta), the dent-like model was found to be appropriate. Basic temperature of seed germination for the three tested varieties was estimated at 5°C. Lower optimum temperatures of seed germination for Hamedan, Malayer and Isfahan varieties were 12.53, 12.75 and 13.45°C, while upper optimum temperatures were 25.00, 32.81 and 33.56°C, respectively. Ceiling temperatures were estimated at 40.12, 42.00 and 35.57°C, respectively. Biological time for 50% seed germination for Hamedan, Malayer and Isfahan varieties was estimated at 165.5, 168.4 and 158.3 hours, respectively.