By virtus of the development of monitoring system and increasing tendency of critically ill patients. We have performed cannulation of radial artery after Allen's test for the continuous monitoring of arterial pressure. It is very important to measure blood pressure, it gives un many informations in the poor risk patients. During monitoring of blood pressure for several years, we have had much troubles because of damping of arteiral pressure waved due to air bubbles. In this study, we used P25Db pressure tranducer and Spaculab Alpha System for monitering of blood pressure, following cannulation of radial artery. Air bubbles of 0.5ml, 1.0ml, 1.5ml and 2.0ml were inserted into monitering system, i.e. fluid filled catheter respectively. The statistical analysis by paired student T-test showed following results. 1) Systolic blood pressure were decreased significantly from 140±14.4 torr of control group to 105±15.9 torr, 102 ±16.1 torr, 99±16.2 torr and 99±15.2 torr in 0.5ml, 1.0ml, 1.5ml and 2.0 ml air groups respectively. 2) Diastolic blood pressure were increased from 70±12.4 torr of control group to 72±13.8 torr, 76±13.9 torr, 78±14.3 torr and 81 ±13.1 torr in each cases respectively. 3) Pulse pressure were decreased from 40±13.3 torr of control group to 33±15.4 torr, 26 ±13.6 torr, 22±13.0 torr and 18±12.0 torr in each cases respectively. 4) But mean arterial pressure showed no significant changes except in 2.0ml air group. Where showed slight change.