The photothermal therapy which mainly uses near-infrared laser to heat tissue has become a new validated method for tumor treatment through direct surface irradiating or inserting light fibers in tumor. The near-infrared light transfer in tissue is limit and the thermal effect occurs only in a small volume during treatment. Thus the treating extent is difficult to control. Otherwise, it is difficult to distinguish the boundary of tumor and normal tissue in photothermal therapy. Improper heating will hurt the normal tissue. So it is important to guide the laser in time in thermal therapy. The results indicate that by using upconversion nanoparticles[Yb/Er (NaYF4Yb, Er)] to localize on tumor, the infrared light can be monitored in real time with the upconversion fluorescence. The light dosage, distribution, heating extent and the boundary of tumor can be detected to increase the effect of thermal therapy.