K-Files are used to trim root canals and enlarge canal after debridement with a reamer. The k file is square in cross section. K files has moderate cutting force. For all preparation techniques with filing motion. For the enlargement of severely curved canals. K files can be divided into Stainless Steel and Nickel Titanium files according to the material. The difference between stainless steel and nickel-titanium files: Stainless steel files are not elastic, while nickel-titanium files are flexible and are more suitable for curved root canals . Stainless steel files have more cutting force, but less breaking resistance. Compared with the NITI files is easy to break, and NITI files is not easy to break, which is safer. However, the cutting force of nickel-titanium files is weaker and less efficient than stainless steel files. The minimum size of stainless steel K file is #06, and we usually only produce more than #8 NITI files. Because the NITI files of #6 is too thin and soft, it is easy to break, and the efficiency is not high. Our hand use NITI files handles have the word “NITi” printed on them.