Diamond has a great potential to be used in high-power, high-voltage and high-frequency semiconductor devices due to its wide band gap (5.5 eV), high breakdown field (> 10 MV/cm), high thermal conductivity (22 W/(cm·K)), and good carrier transport property. High-quality polycrystal diamond with large size wafers (up to several inches) is more easily obtained than the expensive monocrystal diamond plate with the size of only several mm2, and the good performance of electronic device on polycrystal diamond has been reported. So we fabricate a normally-on hydrogen-terminated polycrystal diamond field effect transistor with a 4-μm aluminum gate by using a gold mask process. The saturation drain current is 160 mA/mm, and the on-resistance is as low as 37.85 Ω ·mm. The maximum transconductance reaches 32 mS/mm, and the gate voltage range with the transconductance higher than 90% of its maximum value reaches 3 V (-2 V ≤ VGS ≤ -5 V). An Ohmic contact resistance of 5.52 Ω ·mm and a quite low square resistance of 5.71 kΩ/sq for the hydrogen-terminated diamond are extracted from the analysis of transmission line model measurement. On the basis of the analyses of the obtained results, the on-resistance of device dependent on gate voltage, and the capacitance-voltage data measured at the gate-source diode, we find that the hole sheet density under the gate reaches 1.56×1013 cm-2 at a gate voltage of -5 V, and the extracted effective mobility of the holes stays at about 170 cm2/(V·s) in the afore-mentioned gate voltage range with high transconductance. In summary, the high and broad transconductance peak and the low on-resistance are attributed to the relatively low gate-source and gate-drain series resistance, the high-density carriers in the channel, and the high-level mobility achieved over a large gate voltage range. The relevant research of finding proper dielectrics for the gate insulator and the passivation layer is under way to further improve the device performance.