During routine dental procedures, performing aerosols generating procedure is sometime inevitably. For example, scaling with ultrasonic machine generate both aerosols and droplets which highly contaminate with microorganisms that post a risk to dental staffs. The objectives of this study were to compare the bacterial aerosol contamination during ultrasonic scaling by using magnetostrictive scaler and piezoelectric scaler and to evaluate the duration of the remaining aerosol contamination after using both types of them. Ten subjects were recruited from staffs and patients at Institute of Dentistry, Department of Medical Services, Ministry of Public Health. A split-mouth design was utilized and two treatments were performed on each subject in the same size of two separated rooms: 1) using magnetostrictve scaler; and 2) using piezoelectric scaler. The right or left side of the subject’s mouth was randomly assigned to one of the two treatments. Blood agar plates were placed and exposed to the air for 15 minutes at 3 sites: A) patient’s chest areas, B) dentist’s cabinets, and C) dental assistant’s cabinets. These setting plates were collected at 4 intervals: before ultrasonic scaling 30 minutes, during ultrasonic scaling and after ultrasonic scaling 30 and 60 minutes. All plates were incubated at 35oC for 72 hours and counted for colony forming units (CFUs). The results showed significant increasing in the mean of bacterial counts during using both types of ultrasonic scalers at all sites (p<0.05). The using magnetostrictive scaler presented a statistically significant greater mean of bacterial counts than using 
 piezoelectric scaler at all sites (p= 0.012, 0.036, and 0.036). In addition, the number of bacterial counts was the highest at the patient’s chest areas (99.50 CFUs). The average bacterial counts from using both types of ultrasonic scalers were significantly reduced at 30 and 60 minutes after ultrasonic scaling (p<0.05). This study indicates that the magnetostrictive ultrasonic scaler produces the bacterial aerosol contamination greater than the piezoelectric ultrasonic scaler. Moreover, both types of these ultrasonic scalers are dramatically decreased at 30 minutes after ultrasonic scaling (p<0.05).