Abstract: Objective To evaluate the effects of continued or intermittent irradiation from the diode laser to the root canals in different power, observe temperature changes on the root surfaces in three different irradiation modes. Methods Part 1: a total of 60 single-root human teeth were divided into tow groups, A(power is 1.5w) and B (power is 3 W ). Every group was divided into three subgroups(A1/A2/A3) with 10 teeth in each. Use three different irradiation modes, A1: irradiating for 5 s for 4 times with time interval of 5s, A2: irradiating for 10s for 2 times with time interval of 5s, A3: irradiating continuously for 20s. The temperatures of root surfaces were recorded. Part 2: a total of 30 single-root human teeth were divided into three Groups. Continued irradiation for 20 s was made in power of 1.5 W. Temperatures on three spots of the root surfaces were recorded, G1(apical spot), G2(apical third spot), G3(middle third spot). Results: In group A, A1,A2, and A3 were dealt in different modes, and temperatures increased averagely by 4.78 ℃, 5.21 ℃,5.97 ℃. No significant difference was shown among the three groups. Group B was dealt under the same irradiation condition. The average temperature increase was higher in B than that in grup A. The highest temperature reached 10.3 ℃ in A3. Under the same irradiation condition, the temperature was obviously lower on apical spot(G1) than on apical third spot (G2) and on middle third spot(G3) (P< 0.05). No significant difference was found between G2 and G3. Conclusions The modes of irradiation and power of irradiation both have significant effects on temperature changes on the root surfaces. Changes were different on different spots. The three irradiation modes in power of 1.5 W are all the safe parameters in clinical use.