This study was descriptive research. The purposes were to investigate knowledge, attitude and practice about disinfection of medical instruments and to examine the correlation between knowledge and practice, attitude and practice about disinfection of medical instruments by boiling and using disinfectants. The study subjects were 41 nurse aides in Srisaket Hospital Srisaket province, were obtained by stratified random sampling. Data were collected via questionnaires and observation form which were developed by researcher. The data were analyzed by using descriptive statistics, Pearson product moment correlation coefficient and t-test. The results of this study revealed that 53.7% of nurse aides had poor level of knowledge about disinfection of medical instruments. Sixty one percent had good level of attitude about disinfection of medical instruments, 87.8% had malpractice or did not perform by boiling, 90.2% had malpractice or did not perform by using disinfectants. There was no correlation between knowledge and practice and also no correlation between attitude and practice about disinfection of medical instruments. The suggestion enabled from this study was that nurse aides should be improved knowledge, attitude and practice about disinfection of medical instruments for highly effective disinfection of medical instruments.