In this study, we investigated the synaptic transistors with single- and double-gate structures with Pt/HfOx/n-IGZO MOS stack. Similar to the two-terminal memristor-based synaptic device, the three- and four-terminal transistors exhibited the synaptic motion with drain current modulation through memcapacitance and channel conductance change. The memcapacitance, i.e. a memorized capacitance change, in Pt/HfOx/n-IGZO was analog, polarity-dependent, reversible and nonvolatile so that the drain current with this gate stack was altered as repeatedly applying drain and gate voltages. In addition, the channel conductance change resulting from the ionic interaction between HfOx and IGZO was clearly observed. The modulated drain current in transistor with this MOS stack corresponds to the synaptic weight modulation particularly with analog potentiation, depression, pulse amplitude, width, number, time interval dependent characteristics, which emulate various synaptic motions. In single-gate transistor, the drain current was gradually increased by up to three orders of magnitude as repeating application of gate voltage. Also, its increase was proportional to the positive voltage pulse amplitude and width. Reversibly, the decrease of current was also proportional to the negative voltage pulse amplitude and width. In addition, the current change was changed from volatile to nonvolatile feature as repeating pulses with high amplitude and short interval between pulsing, referring the short-term to long-term transition. Besides the single-gate synaptic transistor altering and measuring the synaptic weight modulation with the same gate, the double-gate synaptic transistor was fabricated and its synaptic motions were demonstrated. Even if the single-gate transistor could modulate the channel conductance; it might have disturbance of synapse state as operating signal processing and learning by the same single gate voltage. From this point, it is beneficial to use double gates; one for learning and the other for signal processing or reading, at the sacrifice of little more complexity in device structure. The channel conductance was altered by applying the voltage in Pt/HfOx/n-IGZO stack and it was measured with Al/YSZ/SiO2/n-IGZO stack that did not alter the conductance but just measured it for reading operation. It verified the separate synaptic weight modulation and reading without disturbance, providing wider synaptic operation schemes with double-gate transistor. The various synaptic motions with single- and double-gate synaptic transistor will be discussed in detail.