Abstract:The breast height form factor (f(1.3)) is a subset on a series of the normal form factor (fn), which has the specialities of fn. Based on the mathematic derivation and calculation of a felled tree, the relationships between the specialities of fn .and relative height (n), form index (r), and between the specialities of f(1.3) and total height (h), r are as follows: 1. To the same r,fn, increases with increasing n, however f(1.3) decreases as h rises. 2. When 0. 03≤n≤0. 22 for fn. To the same n, 5. 87≤h≤43. 33m for f(1.3) to the same h,the larger the value of fn. And f(1.3), is the plumper the stem form is so that the stem form may be reflected. fn and f(1.3) are used as conversion factors for volume; When 0. 22<n≤0. 37 the same n for fn, 3. 51≤h<5. 87m, the same h for f(1.3) only apply to conversion factors for volume; When 0. 37<n<1for j fn, or 1. 3<h<3. 51m for f(1.3), a fraction of fn. Or f(1.3) (it is various in different tree species) is used as a conversion factor for volume, but most ofthem are not ideal for applying to the reflection of stem form or conversion factors for volume.