It is important for improving the theoretical basis of power system voltage stability to demarcate the load stability and voltage stability. Based on the non-linear dynamical system stability theory， the load stability is defined and the boundedness of load instability is found， i.e.， after load instability， the equivalent admittance module only changes within limited range. The essential attribute of voltage stability is studied， and it is proposed that the voltage stability belongs to the problem of output variable boundedness. The relationship between the load stability and voltage stability is analyzed by using Thevenin theorem. The results indicate that the load stability is not equal to the voltage stability and the load instability is not the sufficient condition for the voltage instability determined by the boundedness of load instability. According to the essential attribute of voltage stability， the voltage stability not only relates to the load stability， but also relates to the ability of maintaining voltage of elements and control devices of power system. Simulation results of a typical testing system confirm the validity of the conclusions.