Organic memory transistors based on an organic field-effect transistor (OFET) structure were fabricated by employing a water-soluble polar polymer, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA). The thickness of PAMPSA films was varied from 180 nm to 1000 nm and thermally annealed (treated) at 170 ℃ for 30 min. The annealed PAMPSA films were optically transparent with naked eyes even though the absorbance at the wavelength range of ca. 190~260 nm gradually increased with the film thickness. The devices with the annealed PAMPSA films showed p-channel transistor characteristics at low operation voltages (0~-5 V) and delivered hysteresis of drain current due to the carbon radical-induced dipoles in the thermally annealed PAMPSA films. The best hysteresis characteristics were obtained at the film thickness of 450 nm, whereas the drain current was gradually decreased with the thickness of PAMPSA films. This result has been assigned to the trade-off effect between the capacitance decrease and the dipole increase the PAMPSA thickness increases. The optimized memory devices with the 450 nm-thick PAMPSA layers disclosed excellent retention characteristics during >10,000 cycles of writing-reading-erasing-reading memory tests.