Abstract: On November 7, 1976, a strike slip fault type earthquake of magnitude 6.7 occurred in the border region of the two southwestern provinces, Sichuan and Yunnan, at the localities Ninglang and Yanyuan, About one year before the earthquake, the authors made a medium term prediction for the magnitude and epicenter of an impending strong earthquake, based on the result of analysis of the variation of seismic wave velocity ratio of a region of about 20,000 km2 in extent. The prediction is considered as more or less satisfactong. Of course, the imminent prediction of the time of occurrence was based on other precursory phenomena observed in the region.The results given in this article, may be taken as a new example, for the appearance of variation of velocity ratio before a strike-slip fault type earthquake. A preliminary discussion has been made on the results in consideration of the difference between shear fracturing and strike-slip movements as well as the anisotropy caused by dilatancy.