Vitrinite reflectances may be described by vectors starting from the originof three-dimensional coordinate-system.The length of a reflectance vectordepends on its direction.Theoretically

any vitrinite particle has infinite reflec-tance vectors.These vectors construct a three-dimensional figure called“vitrinitereflectance solid”（VRS）.The three principal axes of VRS are maximum ref-lectance R

max

intermediate reflectance R

int

and minimum reflectance R

min

res-pectively.The VRS may be double concave

and is not a standard ellipsoidof geometry.This paper deals with the geometrical features of VRS in detail

and provi-des a method for determining the lengths and directions of the principal axesby six readings measured on three mutually perpendicular polished-surface.This method may be used for the studies of petroleum geochemistry

coalpetrology and structural geology.