Abstract: Steady state strength is an important parameter in evaluating the stability of sand deposits against flow liquefaction. Most loose sands exhibit the quasi steady state (QSS) behavior under undrained triaxial loading, in which no unique ultimate steady state is reached. Stress controlled triaxial tests were carried out on Unimin sand to study the steady state strength in the QSS behavior. The test results indicate that there are usually four distinct deformation stages in undrained tests on loose sand: initial stage, collapse stage, critical stress stage and post failure stage. The strain rate approximately remains constant in each deformation stage. The steady state, in which pore pressure, deviator stress, effective stress and strain rate remain constant, occurs in the collapse stage. The conventional definition of the steady state of deformation is not completely valid for the QSS behavior, and a modified definition is suggested.