In this paper we use the Scaled Serial Fraction (SSF) analysis to study the scalaility of three parallel sort algorithms and that of the algorithms on a hypercube architecture. The scalability is a very important metric for predicting the performance of a parallel algorithm of a parallel system if we consider a massively parallel system with thousands of processors. The condition is given under which the three parallel sort algorithms are all scalable and explain the applicability range of each algorithm. We show that by scalability analysis one is able to exhibit the essential properties of parallel sort algorithms in a massively parallel processing environment. Finally the computed results of the scaled serial fraction, the scaled efficiency and the scaled speedup are given.