The property and behavior of twin roll thin strip is greatly affected by three dimension size of columnar and equiaxed crystal grains in the solidification structure of thin strip The growth of crystal grain has three dimension feature during casting process of thin strip, So the token of three dimension size of crystal grain cannot be realized by tradtional token method. On the basis of assumption to crystal grain shape, the token expression of three dimension size of columnar is established along with crystal grain in microstructure of twin roll thin strip according to quantitative metallurgical analysis and probability theory, And quantitative discription about three dimension growth feature of crystal grain in microstructure of twin roll thin strip is realized.