The temperature dependence of permittivity in [111]-oriented 0.76PMN-0.24PT single crystal revealed dielectric singularity between 0 and 100°C under a dc bias field (Edc). By fitting the singular permittivity-temperature characteristics under increasing dc bias fields, it was found that the critical exponent γ, is a function of Edc and increased from 0.2 to 0.8 at Edc ≤150 kV m, to 1.22 at Edc =175 kV/m, which is approaching the value of the three-dimensional (3D) Ising model (1.25), and then to 1.84 at Edc =200 kV/m, close to the value of the 3D random-field Ising model (1.97). The maximum tunability of 80% was obtained at the temperature when a relaxor ferroelectric (RF) to normal ferroelectric (FE) phase transition occurs. On the other hand, the maximum tunability was only 25% at the temperature when a paraelectric (PE) to RF phase transition occurs. In addition, the PE-RF transition temperature Tmax was almost independent of the dc field, whereas the RF-FE transition temperature Td had a relation of Td ∼ E2/3 at E>50 kV/m. © 2005 The American Physical Society.