Objective To analyze the effect of five kinds of complicated root canals therapy by utilizing sur- gical microscopes.Methods A total of 359 exactly diagnosed complicated root canals were consecutively treated from July 2003 to December 2004 in department of stomatology,No.4 Affilicated Hospital of Inner Mongolia Medical College.The complicated root canals observed included calcified blocked root canals,instrument fracture root canals, MB_2 root canals,"C"type root canals and retreated root canals.Two groups were randomly departed according to the patients' will.180 complicated root canals were in the therapy group treated with surgical microscopes;in control group,179 complicated root canals were treated by traditional root therapy methods without surgical microscopes. From two aspects of statisties and clinic,two groups' achievement ratios were compared.Results Statistically achievement ratios of five kinds of root canals in the therapy group were 68.18 %,71.43 %,70.00 %,83.33 %, 80.39 % respectively,higher than those in the control group which were 13.33 %,16.67 %,25.00 %,22.73 % and 38.46 % respectively.The differences had obvious statistical significance(x~2=21.607,23.572,8.120,19.100, 20.488;P=0.000,0.000,0.004,0.000,0.000,P