Instability of metal-oxide-semiconductor field-effect transistor (MOSFET) characteristics was evaluated by DC and pulse current-voltage (I-V) measurements. MOSFETs with nirided gate oxides were fabricated on C-face 4H-SiC. Their interfaces have near interface traps (NITs) with long time constants, depending on the cooling down process after nitridation. Such devices exhibited a large hysteresis in DC I-V and a large transient current in pulse I-V measurements. These phenomena can be explained by the charge state of NITs due to capture/emission of electrons in the channel.