Gap frequency is the probability that a light unintercepted penetrates through the canopy and reaches the surface under the vegetation

and it is a key variable describing canopy structure and biomass spatial distribution.Previous study revealed that in the night TIR images

the brightness components of maize field was stable and easy to be distinguished.The objective of the research is to investigate the directional variation of gap fraction over a maize canopy（LAI=3.64） by night thermal infrared（TIR） experimental data which was acquired on August 23rd

2005 in Huailai county of Hebei Province.The results showed that very little azimuth variation appears for gap frequency except for the observations along row direction

gap frequency declines from the nadir slowly;in other azimuth directions

gap frequency declines sharply.The measured parameters were input into a GORT model

and the gap fractions were simulated at different angles.By comparing the simulated gap frequency with the measured one

the two had good agreement.The model caught the main features of the measured gap fraction of maize canopy and reflected the characteristics of row crop.The differences between them were due to some reasons

such as the disagreement of the objects;the angle variation was not controlled accurately in experiment;the measurement height was too low;the selection of projection function and clumping parameter et al.