Objective To analyze the differences of clinical results between CAS and routine NCAS localization in TKA and to evaluate the safety and reliability of CAS in TKA in order to take advantages and avoid disadvantages of this new technology.Methods From October of 2004 to October of 2005,18 cases(20 joints)of TKA in Group Ⅰ which were perform utilizing computer assisted imaging guidance system(CAS)and there were 7 males and 11 females with an average age of 67 years.From October of 2003 to October of 2004,17 cases(22 joints)of TKA in Group Ⅱ which were performed with conventional method previously(NCAS)and there were 5 males and 12 females with an average age of 65 years(56-78).The differences between two groups in clinical results,mechanical axes,soft-tissues balance,coincidence,blood loss,and operating time were statistically analyzed.Results All cases were followed for an average of 10 months(3-12).The mechanical axes error was 2-3 degrees in CAS group and 3-6 degrees in NCAS group.Soft-tissues balance angle was within 2 degree in CAS group and 2-4 degrees in NCAS group.The distance variable was 2-4 mm in CAS group and 5-7 mm in NCAS group.The blood loss was 550-700 ml in CAS group and 700-900 ml in NCAS group.The surgery time was 75-100 min in CAS group and 45-60 min in NCAS group.There were statistically significant differences between two groups.There were no major complications such as fat embolism occurred in both groups.Conclusion Utilizing computer assisted imaging guidance in TKA can obtain reliable and sound clinical results with better mechanical axes and soft-tissues balance.It is a safe and reliable procedure in TKA.