In this paper, we tried to estimate the effect of control method on floating offshore wind turbine. The experiment in the water basin revealed that traditional blade pitch control can amplify the platform pitch oscillation of floating wind turbine. In order to understand the physical phenomenon, we used aeroelastic simulation using GH Bladed. Turbine model is based on the turbine used in wind tunnel test. To simulate the pitching motion of floating platform, we used onshore wind turbine model with inflow with oscillating wind speed that simulates relative wind speed change from wind turbine's fore-aft pitching motion. Two types of control method are used; fixed pitch variable speed control which represents before rated state of large wind turbines and variable pitch variable speed control which represents over rated state of large wind turbines. Comparing the relation between wind speed change and rotor thrust force change of two control methods, we made it clear that traditional blade pitch control method make thrust force change almost the inverse of wind speed increase and decrease. From thrust force inverse to wind speed change, tower pitching motion can be amplified. That is, blade pitch control can induce negative damping on tower pitching motion. As a conclusion pitch control can increase larger blade load although pitch control aims to reduce the blade load. (C) 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SINTEF Energi AS. Open access under CC BY-NC-ND license.