This study was conducted to examine the dry matter production and leaf photosynthesis under different fertilizer application in Yacon(Polymnia sonchifolia)plants. Under field conditions, dry matter production of stable mature application plot(S-1) was much larger than non-treated control plot(CTL), but bulb dry weight of 10kg nitrogen application plot per 10a (N-10) was not differentiated from CTL. The maximum photosynthetic rates increased in Yacon plants grown at S-1 while in the rates of CTL reduced under glasshouse in the same treated pot condition. The optimum temperature for the highly photosynthetic rates is about $34{pm}3^{circ}C$, and the rates did decrease by stomatal resistance below 28℃. The estimated light saturation point was $1200{mu}mol{cdot}m^{-2}s^1$. The results of this study indicate that soil conditions were highly related to dry matter production and leaf photosynthesis rate in Yacon plants.