The effects of processing parameters on phase formation of hydrothermally synthesized bismuth ferrites were investigated. Bi25FeO40, BiFeO3, and Bi2Fe4O9 with different morphologies and grain size were synthesized by dissolving bismuth nitrate and iron nitrate in KOH solution at reaction time ranging from 3 to 20 h. The subcrystallized part of BiFeO3 obtained with reaction time of 4 and 5 h displayed strong saturation magnetization of 2.914 emu/g. The results of the electron spin resonance (ESR) indicated each unpaired spin providing a net magnetic moment which eventually led to the enhancement of the saturated magnetization. Furthermore, Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves exhibited spin-glass freezing behavior in all the samples. And the low-temperature magnetic hysteresis loop displayed the ferromagnetism of BiFeO3 with different structures and the phenomenon of exchange bias, respectively.