At present

determining the rock joint size is usually based on the joint trace length in exposed rock area

but the estimated results are often affected by the limited area of exposed rock mass. In this research

we try to estimate the rock mass size according to the joint characteristics around a borehole. At first

the joint and dip information in a geological drill is obtained through the borehole camera technology

a equation for joint plane is developed in the range of drilling depth. Based on this equation

the connectivity criterion among the joint surfaces is constructed and the joint surface trace length is determined; subsequently

the trace mean function of trace length. Finally

the size of rock joint is estimated according to the relationship between trace length and the rock mass size.This method is applied to the combined underground mining-ore dressing of Zhangjiawan iron ore

showing its reability and high accuracy.