In this chapter, we forecast the spectral state transition (hard to soft state) theoretically for the transient sources at a constant accretion rate. We first demonstrate a systematic evolution of the relative size of the Comptonizing medium, supposed to be an independent second parameter (accretion rate be the first) for the harder to softer spectral transition. The second parameter is also essential in obtaining the hysteresis of the hardness ratio during outburst and to procure the “q” diagram theoretically. We therefore study extensively the variation of the emitted post-shock radiation in the relativistic accretion flow harboring a dissipative shock. With the dissipation, as the shock moves inward, the inner edge of the disk reduces progressively and attains a minimum for maximum dissipation where transition occurs. The extreme shock location determines the minimum corona size which is obtained around \(10 \ r_g\ (=\frac {GM}{c^2})\) for moderate spin. The present chapter is therefore significantly important for the black hole X-ray binary (BHXRBs) sources to examine the origin of spectral transitions, and QPO evolution etc.