One of the serious challenges encountered in cementing oil and gas wells is the failure of the cement sheaths and its debonding from casing or formation rock. Shrinkage of the cement during setting is identified as one of the driving factors behind these issues. Some expansive cement systems have been developed in the oil and gas industry to compensate for the shrinkage effect. All the expansive additives which have been developed so far have chemical reactions with the cement itself that would significantly impact the mechanical strength of the cement. In this paper, we present a new class of polymer-based expandable cement additive particles which are made of shape memory polymers (SMP). This class of polymers is deigned to expand to the required extent when exposed to temperatures above 50-100°C which is below the temperature of the cementing zone. It is notable that expansion occurs after placement of the cement but before its setting. The API RP 10 B-2 & 5 have been followed as standard test methods to evaluate expansion and strength of the cement slurry after utilizing the new additive. The proposed additive does not react with the water or cement content of the slurry. Mechanical evaluation tests confirm the high potential benefit of this additive without any deteriorative effect on mechanical properties or setting time of the cement paste and significant impact on its mechanical properties. Hence this additive would provide a reliable way to prevent cement channeling, debonding and fluid migration to upper formations.