Abstract: Using the viewpoint of fracture mechanics, a relation between earthquake magnitude M, shear stress τ₀ of the source region and radius a of the earthquake fracture (assumed eircular) was obtained previously, from which one can get the stress value τ₀ if a and M are known. The direction of the stress field is determined by averaging the focal mechanism solutions of small earthquakes of the region. This paper discusses the variation of stress field τ₀ with time for the source region before and after the Yan-Yuan Earthquake, Yunnan Province, occurred on November 7, 1976, magnitude 6.9 and compares the stress field with that of the neighbouring regions.The results are as follows:(1). Before the Yan-Yuan Earthquake, in the neighbouring region Zhong-Dian, the value of τ₀ was four times higher than all other parts of the entire province of Yunnan.(2). In the Yan-Yuan region, from the beginning of 1976, the τ₀ value increased substantially and meanwhile the average direction of stress field from small earthquake source mechanism solutions was essentially the same as that of the main shock. Thus, one can see the actual variation of the stress field before the Yan-Yuan Earthquake.A discussion regarding the errors and sources of errors in the computation of the τ₀ valus has been made and also the possibility and the consequent error for comparing the τ₀ values of different regions using a single recording station or many stations, analysed.