Abstract: A near infrared light test apparatus, which used LEDs of a wavelength of 940 nm as its light source, was designed for precisely measuring the number of cotton fibers. Eight cotton fiber samples of different harvest years and different color grades were selected for measuring the fiber numbers and the corresponding relative attenuation of near infrared light(NIR). The results show that the property of attenuation of cotton fiber by NIR is correlated with the input of light intensity. The relative attenuation decrease as input increases for the same cotton. The analysis of the data shows that the number of cotton fiber is the positive exponent function of relative attenuation by the cotton, and the equation is much better than classic Lambert’s law for cotton fiber property of light. The number of cotton fiber can be well estimated from the equation with R2 of 0.99 at a confidence level of 95%. The effects of harvest years and color grades were insignificant for the property of NIR light of cotton fiber.