Aim: The aim of the study was to determine the influence of the sterilization on the fracture behaviour nickel- titanium endodontic instruments (Mtwo 25/ 06) in two different standardised artificial root canals. Material and method: The investigation was realised in 80 new nickel- titanium files of the size 25/06. In each case 20 files were submitted to a different number by disinfection- and sterilization cycles (0, 5, 10, 15 cycles). Afterwards the files were introduced with the help of a standardised experiment set-up in two differently very curved, artificial root canals up to a working length of 16 mm. All files were loaded with a steady speed (300 U/min) up to the break of the file. The time (seconds) up to the fracture was determined. The statistical evaluation of the results of eight test groups was carried out with ANOVA and Tukey's test (SPSS 17.0) Results: With the very curved root canal it has come after 107,7 (± 14,4) seconds to the fracture of the new files. After the sterilization the following values were determined: 5 cycles: 90,9 (± 9,4); 10 cycles: 76,6 (± 10,1); 15 cycles: 69,9 (± 5,1). The less curved root canal showed the following values: 0 cycles: 332,9 (±40,1); 5 cycles: 291,4 (± 29,2); 10 cycles: 289,6 (± 26,5); 15 cycles: 273,7(±38,7). The statistical evaluation showed a significant influence of the canal bend on the fracture behavior (p 0,001, ANOVA). The influence of the sterilization was significant with both canal forms in comparison to the unsterilised files. After the different number of sterilization cycles no significant difference of the fracture time could be observed with both canal forms. Conclusion: It can be found out that the fracture time is influenced, on the one hand, significantly by the canal bend and on the other hand decisively by the first sterilization cycles. Keywords: root canals, nickel-titanium instruments, sterilization