New irradiation capsule for the research reactor was developed using a radiation resistant polymer, Polyethylene-2, 6-Naphthalate (PEN). This capsule has several merits such as low activity after irradiation, low price by mass production using injection moulding, and good irradiation performance for application to 12 hours irradiation in Japan Research Reactor No. 4 (JRR-4). Using this capsule, more efficient irradiation can be done because the sample can be taken off from the capsule immediately after irradiation, and this PEN capsule can be replaced with ordinary aluminum capsule for JRR-4 irradiation. This report describes the development of PEN capsule and the irradiation tests. (author).