The electrocaloric effect (ECE) of poly (vinyledene fluoride–trifluoroethylene) (P(VDF–TrFE)) 55/45 mol% copolymers was directly measured, which confirms the results deduced from Maxwell relation. The adiabatic temperature change ΔT under a given electric field peaks at the ferroelectric–paraelectric (FE–PE) transition. Away from it, ECE becomes small. ΔT versus applied electric field can be described well by a modified Belov–Goryaga equation. The ECE in ferroelectric polymers, especially near FE–PE transition where larger ECE is observed, are analyzed under different boundary conditions employing phenomenological theory and constitutive equations. The secondary pyroelectricity is found to play a significant role which enhances ECE in ferroelectric polymers.