When people open their eyes normally, the regions of iris will be covered by eyelids and eyelashes. We investigate the performance of the use of a partial iris for recognition by using normalized correlation matching method in different iris image databases. The experimental results show that the inner rings of the iris over 37.5% of iris pattern exposure,or the angle of the sector exposed over 70° would be necessary for enough information in human verification. The results also show the curve of the relationship between percentage of the iris used and the accuracy rate.