Geometric characteristics of forest gaps involving gap size, gap shape and canopy height are major determinants of environmental heterogeneity within gap and then affect plant regeneration and succession. Therefore, quickly measuring methods of the three gap properties lay the foundations for studies of forest gaps. The methods for measuring gap geometric properties can be classified into two kinds: ground\|based and photographic. Ground\|based methods are based on actual measurement on the ground, while photographic methods are based on the photographs of canopies of canopy trees surrounding the gap. The ground\|based methods are viable for both canopy gap size and expanded gap size but are time\|consuming and arbitrary, especially for canopy gap size. Moreover, none of the ground\|based methods is viable for measuring gap shape or the height of canopy trees surrounding the gap. Compared with the ground\|based methods, the photographic methods are more objective and less time\|consuming, but all are only suitable for canopy gaps. Of five present photographic methods, the earlier three ones are only viable for canopy gap size. The improved hemispherical photograph method can measure canopy gap size and shape at the same time. Compared with the earlier three ones, the improved hemispherical photograph method is more accurate but needs more parameters. Among all methods for measuring gap geometric characteristics, the two hemispherical photographs (THP) method is the only method that is suitable for measuring gap size, shape and canopy heights at any azimuth (i.e., tridimensional shape of canopy gap) and has good accuracy. In conclusion, THP is a rapid, simple, objective and accurate approach to the determination of tridimensional shape of canopy gap. These advantages make THP a powerful tool for repeatable, comparative, long\|term studies of canopy gaps. It is essential to study environmental heterogeneity and plant regeneration within gap taking gap size, shape and canopy heights into consideration at the same time.