Visualization experimental investigation of pulsating heat pipes (PHP) was conducted. A PHP was charged with distilled water, TiO₂/H₂O nanofluid and CuO/H^₂O nanofluid as working fluids by a filling ratio of 55%, respectively. Both the nanofluids were made of distilled water and 1% volume fraction of nanoparticles. Experimental results showed that nanoparticles settled down in the motionless base liquid, but the oscillating motion in PHP would excite them to suspend them. The suspending stability of nanofluid got weak as temperature increased. Compared with the distilled water PHP, nanofluid PHP could work in lower heat loads, and had shorter startup time, lower thermal resistance, and temperature oscillation of smaller amplitude and higher frequency. At the working temperature of 110℃, the thermal resistance of PHP was 0.23℃/W, when it was filled with distilled water, and the thermal resistance would go down to 0.11℃/W or 0.13℃/W, when it was charged with TiO₂/H₂O or CuO/H₂O. The two nanofluid PHPs had similar heat transfer performance.