In this work, reactions of phenol were carried out in supercritical argon (critical temperature, Tc: 150.7 K, critical pressure, Pc: 4.8 MPa) with pulsed discharge plasma to understand reaction characteristics and to evaluate possibility that this technique will be applicable for a new “green” polymerization technique of functional polymeric materials. Experiments in subcritical water or in supercritical argon were conducted through the operation of a specially-designed SUS316 batch-type reactor (inner volume: 900 mL) at 373-523 K and 1-25 MPa, or at 303-373 K and 5-15 MPa, respectively. The electrode configuration consisted of a point (negative electrode) and a planar surface (positive electrode), which were made of tungsten and stainless steel, respectively. The distance between the two electrodes was fixed at 1 mm. Two kinds of power supply devices (BPFN and MPC) were employed. As results using a BPFN, it was found that reaction behavior in subcritical water at 373-523 K, 1-25 MPa with less than 4000 times pulsed discharges basically similar to that in supercritical argon, but polymerized products of phenol could be obtained under larger pulsed discharge times like 5000 times at identical conditions. In contrast, phenol could be converted into hydroquinone but no polymerized product could be confirmed in supercritical argon. On the surface of the electrode used, it was found that phenol could be converted into amorphous graphite oxide with pulsed discharge plasma treatment in supercritical argon. This finding will be expected as a new method for the carbon-based functional materials in supercritical argon.