The errors of determining direction of fracture plane and environment stress of moderate and small earthquakes are analyzed in detail.The result shows that the main factors causing the error of determining direction of fracture plane are the field angle in which the stations are distributed and the precision of making the theoretical master for the generalized directional function.If the theoretical curve is given for every 15°of θ, then the error can be less than 15°when the field angle is more than 60°.The main factor creating the relative error of the environment stress τ₀ is the magnitude error.If the magnitude error is 0.3,it may cause the relative error of the τ₀ to reach 70%