Thermal analysis and Raman spectra were carried out in SrBi2Ta2O9 (SBT) nanoparticles to investigate phase transitions. Two anomalies have been observed in temperature dependence of specific heat for SBT nanoparticles. Under the combination with Raman spectra, it indicates that there exists a new ferroelectric intermediate phase in the phase-transition sequence. So we can conclude that the phase-transition sequence in SBT nanoparticles should be ferroelectric-ferroelectric-paraelectric. Moreover, the size effect was discussed in consideration of inner compressive stress in nanoparticles for this special transition behavior. The calculated results show that the SBT nanoparticles keep the ferroelectricity until the particle size is decreased to 4.2 nm.