This research paper mainly presents an investigation of the microstructures and magnetic properties of bulk ferromagnetic shape memory (FSM) FePdNi (Ni = 4, 8 at.%) alloys, by transmission electron microscopy (TEM), a magnetostrictive-meter setup, and a superconducting quantum interference device (SQUID) magnetometer. The FSM alloys were homogenized through hot and cold strain forging (SF) to a ∼38 % reduction in thickness, solution-treated (ST) with annealing recrystallization at 1100 °C or 8 h, quenched in ice brine, and then aged at 500 °C for 100 h (5 HTA). The investigation of the microstructures and magnetic properties indicated that the higher Ni content (N = 8 at.%) in the FePdNi alloy SF and ST reduced the saturation magnetostriction at RT. However, with higher Ni content in the FePdNi alloy, the decomposition of L1 + L1 twin phases into stoichiometric L1 + L1 + α structures was suppressed after the alloy was ST and aged at 5 HTA, as confirmed by TEM investigation. The result was that the FSM FePdNi alloy maintained a high saturation magnetostriction and magnetization after the alloy was ST and aged at 5 HTA. This magnetic property of the FePdNi FSM alloy makes it suitable for application in high-temperature ( T < 500 °C) and high-frequency environments. However, low Ni content FSM FePdNi (N = 4 at.%) alloy SF, ST, and aged at 5 HTA underwent decomposition of the L1 + L1 twin phases into the stoichiometric L1 + L1 + α structures, as confirmed by TEM, leading to a decrease of saturation magnetostriction and magnetization. This magnetic property of the FePdNi FSM alloy is not suitable for application in high-temperature ( T < 500 °C) environments. [ABSTRACT FROM AUTHOR]