Effect of thermal arrest on the L21-tetragonal martensitic transformation in a NiCoMnSn shape memory alloy was investigated. The phenomenon was studied by interrupted heating/cooling in differential scanning calorimetry analysis. The experimental evidence indicates that the forward martensitic transformation continued to completion during cooling arrest between Ms and Mf. The same behavior was also observed for the reverse transformation on heating. These observations demonstrate that the L21-tetragonal martensitic transformation in the Ni43Co7Mn41Sn9 alloy is time dependent at the finite cooling rate.