Abstract: Non-associated flow may lead to the instability of sand and non-unique solution of the prob-lem,but neither Drucker’s nor Hill’s postulate gives necessary but sufficient conditions for stability ofgranular materials.Based on the concepts of local yield and plastic potential surface,non-associative be-haviour of saturated sands is analyzed,and a sufficient and necessary condition for the stability is prop-osed.The potential instability line is established on the p’-q plane, which is on the “collapse surface”￣8