Abnormal electrocaloric effect (ECE) is observed in (1-x)Bi0.5Na0.5TiO3-xBaTiO3 (BNT-xBT) solid solutions by calorimetry measurements, manifesting itself by the differences in the heats released and absorbed under the application and withdrawal of electric fields, respectively. The abnormal ECE is explained by the coexistence of positive and negative ECEs [1] close to the ferroelectric-to-antiferroelectric transition temperature, which is demonstrated by the structural and ferroelectric properties determined from anelastic analyses and thermodynamics calculations on the solid solutions under an applied electric field, respectively. A threshold electric field below which the negative ECE occurs is obtained, in comparisons with that predicted by the thermodynamics Maxwell’s relations. The abnormal ECE in the BNT-xBT solid solutions and the inconsistency of ECE between the direct and indirect measurements [2] could be attributed to the formation of a metastable AFE state under an applied electric field.