An experimental study was performed to understand the pool boiling heat transfer of deionized waterbased carbon nanotube (CNT) suspensions on a flat copper surface under atmospheric and subatmospheric pressures. Experimental results indicate that the addition of CNT in the base liquid can apparently enhance both the heat transfer coefficient (HTC) and the critical heat flux (CHF). The pressure has great impacts on both the HTC and the CHF enhancement of CNT suspensions and the both increase significantly with the decrease of the pressure. The CNT mass concentration of CNT suspensions also has strong influences on both the HTC and the CHF of CNT suspensions and there exists an optimal CNT mass concentration corresponding to the maximum heat transfer enhancement. Besides, the effects of the surface characteristics of the heated surface on both the HTC and the CHF of CNT suspensions were also investigated.