Strain-induced phase transformations (PTs) in a sample under compression, unloading, and reloading in a diamond anvil cell are investigated in detail, by applying finite element method. In contrast to previous studies, the kinetic equation includes the pressure range in which both direct and reverse PTs occur simultaneously. Results are compared to the case when “no transformation” region in the pressure range exists instead, for various values of the kinetic parameters and ratios of the yield strengths of low and high pressure phases. Under unloading (which has never been studied before), surprising plastic flow and reverse PT are found, which were neglected in experiments and change interpretation of experimental results. They are caused both by heterogeneous stress redistribution and transformation-induced plasticity. After reloading, the reverse PT continues followed by intense direct PT. However, PT is less pronounced than after initial compression and geometry of transformed zone changes. In particular, a localized transformed band of a weaker high pressure phase does not reappear in comparison with the initial compression. A number of experimental phenomena are reproduced and interpreted.