Prediction of germination time is one of the main goals of seed researchers. Therefore in order to evaluation the cardinal temperatures and the investigation response of promising forage sorghum lines to temperature, this experiment was conducted as factorial in a completely randomized design with three replications, 15 forage sorghum lines and nine fixed temperatures 10, 15, 20, 25, 30, 35, 40, 45 and 50˚C at Seed Technology Laboratory of Islamic Azad University Shoushtar Branch in 2013.               Results of analysis of variance showed that investigated germination components for factor line, temperature and interaction line × temperature were significant. The maximum germination percent-age in temperatures 20, 25 and 30˚C and the most speed rate, uniformity of germination and the minimum duration time till 95 percent the maximum germination (D₉₅₎ in temperature 40˚C was obtained. With temperature drop from 40˚C, germination speed rate and D₉₅ gradually decreased and increased, respectively. The base temperature (T_b) of forage sorghum lines from 9.36˚C (KFS₁₁) to 14.28˚C (KFS₂), optimum temperature (T_o) from 23˚C (KFS₂) to 37.2˚C (KFS₁₁) and maximum temperature (T_c) from 40.7˚C (KFS₂) to 49.76˚C (KFS₁₅) varied. Cluster analysis with ward’s method in the basis 7 studied traits led to three clusters formation. Third cluster included the lines KSF₆, KSF₇, KSF₉ and KSF₁₀ with the maximum percentage, rate and uniformity of germination and the minimum D₉₅ jointly in temperatures 25˚C, 30˚C, 35˚C and partially 40˚C therefore these lines are recommended for supplementary researches.