Aerodynamic forces and power requirements inforward flight in a bumblebee(Bombus terrestris)were stud-ied using the method of computational fluid dynamics.Actualwing kinematic data of free flight were used in the study(thespeed ranges from 0 m/s to 4.5 m/s,advance ratio ranges from0-0.66),The bumblebee employs the delayed stall mecha-nism and the fast pitching-up rotation mechanism to producevertical force and thrust.The leading-edge vortex does notshed in the translatory phase of the half-strokes and is muchmore concentrated than tbat of the fruit fly in a previousstudy.At hovering and low-speed flight,the vertical force isproduced by both the half-strokes and is contributed by winglift;at medium and high speeds,the vertical force is mainlyproduced during the downstroke and is contributed by bothwing lift and wing drag.At all speeds the thrust is mainlyproduced in the upstroke and is contributed by wing drag.The power requirement at low to medium speeds is not verydifferent from that of hovering and is relatively large at thehighest speed(advance ratio 0.66),i.e.the power curve is J-shaped.Except at the highest flight speed,storing energy elas-tically can save power up to 20%-30%.At the highest speed,because of the large increase of aerodynamic torque and theslight decrease of inertial torque(due to the smaller strokeamplitude and stroke frequency used),the power requirementis dominated by aerodynamic power and the eflect of elasticstorage of energy on power requirement is limited. Aerodynamic forces and power requirements in forward flight in a bumblebee (Bombus terrestris) were stud-ied using the method of computational fluid dynamics. Actualwing kinematic data of free flight were used in the study (thespeed ranges from 0 m / s to 4.5 m / s , advance ratio ranges from 0-0.66), The bumblebee employs the delayed stall mecha-nism and the fast pitching-up rotation mechanism to producevertical force and thrust. The leading-edge vortex does notshed in the translatory phase of the half-strokes and is muchmore concentrated than tbat of the fruit fly in a previous study. At hovering and low-speed flight, the vertical force isproduced by both the half-strokes and is contributed by winglift; at medium and high speeds, the vertical force is mainlyproduced during the downstroke and is contributed by bothwing lift and wing drag.At all speeds the thrust is mainlyproduced in the upstroke and is contributed by wing drag. power requirement at low to medium speeds is not very different from that that iethe power curve is J-shaped. Except at the highest flight speed, storing energy elas-tically can save power up to 20% -30% .At the highest speed, because of the large increase of aerodynamic torque and theslight decrease of inertial torque (due to the smaller stroke amplitude and stroke frequency used), the power requirementis dominated by aerodynamic power and the eflect of elastic storage of energy on power requirement is limited.