A direct calorimetry method was developed and used to measure the electrocaloric effect (ECE). A temperature change ΔT of over 20°C and an entropy change ΔS of over 95 J/(kgK) were procured at 33°C and 160 MV/m in the high-energy electron irradiated poly(vinylidene fluoride- trifluoroethylene) (P(VDF-TrFE)) 68/32 mol% copolymers, which were larger than those of terpolymer blends (ΔT = 9°C, ΔS=46 J/(kgK) at 180 MV/m and room temperature) and our earlier report on P(VDF-TrFE) 55/45 mol% normal ferroelectric copolymer (12°C and 55 J/(kgK) at 80°C). We observed that the β value ((8.7±0.6)×10 7 JmC -2K -1) in the equation of ΔS=1/2βΔD 2 derived from ΔS - ΔD 2 relation for irradiated copolymers was larger than that of the terpolymer blends ((5.4±0.5)×10 7 JmC -2K -1). It was also found that the irradiated copolymer showed a sharp depolarization peak at T d < T m (maximum permittivity temperature), which is frequency independent, in the dielectric constant - temperature characteristics, a larger depolarization value at T d in the thermally stimulated depolarization current (TSDC) - temperature relationship, and a larger volume strain/longitudinal strain ratio over terpolymer blends. The giant ECE in irradiated copolymer is regarded as due to the greater randomness present in the relaxor state. In irradiated copolymers, the long all-trans chains are broken by the high-energy electrons, which make the small sized all-trans sequences more easily reorient along the electric field, more remarkably affecting the permittivity, TSDC, and volume strain.