The relationship between Reynolds' number and the coeffecient of air resistance has been studied on mono-filament fibers by measuring the terminal velocity of fibers dropping spontaneously in stationary air. The results obtained are: (1) With Reynolds' number Re of a fiber defined as Re=usd/v by using diameter d of the fiber to represent its size, it is possible to determine experimentally the relationship between the coefficient of air resistance C and Reynolds' number Re as follows: For mono-filament fibers C=24/(2.75Re0.756) For cotton tufts C=24/(193Re2.00) (2) The coefficient of air resistance for a mono-filament fiber is much smaller than Stokes' coefficient for a sphere or Lambs' coefficient for a cylinder, assuming the three to be equal in Reynolds' number. (3) The effect of fiber crimp upon the coefficient of air resistance is so slight, in fact almost negligible.