Objective: To study some characteristics of patients after open heart surgery and evaluate the effectiveness of monitoring direct arterial blood pressure in patients after open heart surgery. Subject and research method: cross-sectional descriptive study performed on 50 patients who were indicated to undergo open heart surgery, postoperation care at the Recovery Ward, Central Military Hospital No.108 from September, 2010 to September, 2011. Result: the proportion of female is higher than that of male (p 0.05). Mean age is 47.15 + or - 3.8, height, weight and BMI of the male is higher than that of female (p 0.05). The effectiveness of nurses closely monitoring the change of direct arterial blood pressure at the moment directly after surgery (T0) and noticing the doctors to handle together achieve the result that arterial blood pressures gradually stabilized 12 hours postoperation (T2). When direct arterial blood pressure has decreased, arrhythmia, tachycardia, bradycardia and atrial fibrillation happened (p 0.05), when CVP 7 cmH2O or 25 cmH2O and when patients suffer blood loss through drainage 750 ml (p 0.05). Conclusion: There is a relation between decreasing direct arterial blood pressure with heart rate, with blood loss through drainage 750 ml (p 0.05). When arterial's blood pressure varies, vasoactive medicines need to be used before the phenomenon of peripheral vasoconstriction occurs (p 0.05).