Since the traffic of NetNews is increasing, keeping of all articles becomes serious problem from a viewpoint of wasting of network bandwidth and the amount of disk usage. In addition, not all incoming articles are read by users. We have proposed several caching algorithms to overcome this problem and shown that a selective prefetch scheme gives the best system performance among the proposed ones. However, since the selective prefetch scheme employed a simple prefetching policy, the scheme gave low hit ratio in some cases.Therefore, this paper intends to improves the selective prefetch scheme from a viewpoint of the amount of disk usage as well as hit ratio. In this paper, we divide the scheme into three factors; reference span, criterion, and threshold in criterion. Through simulation experiments using actual NetNews logs, we investigate the influence of the factors of the reference span and the threshold to system performance. As a result, it is shown; the reference span is more significant factor than the threshold, the selective prefetch scheme with a value around the seven days reference span keeps high hit ratio and reduces the amount of disk usage.