Flutter behaviors of flexible thin sheets of such as papers and plastic films blown in wind, whose mass ratios are very low, tend to deviate far from those for ordinarily stiff materials. A theoretical prediction method of the flutter occurrence developed by the authors was applied to the low mass ratio region, and the results explained well the tendencies found in the experimental data. The predicted results have shown clearly the effects of both the fluid friction and the added mass effect by surrounding fluids in the region for the low mass ratios, compared with those for higher mass ratios whose behaviors are dominated mainly by elasticity and inertia force, and fluid pressure. The flutter limits and the behaviors in a region for very low mass ratios, which has not yet been observed experimentally, were predicted by the method and given in this report.