A method of forming a film comprising nanocellulose having an Oxygen Transmission Rate (OTR) value in the range of 0.1 to 300 cc/m2/24 h at 38° C. and 85% relative humidity (RH), and having a basis weight in the range of in the range of 0.1 to 45 g/m2 wherein the method comprises the steps of; providing a suspension comprising nanocellulose, forming at least one layer of a web or a film from said suspension; drying said formed web or film to a dry content of at least 65 weight-%, wherein said method further comprises the steps of; treating at least one side of said dewatered and dried web or film with ultra violet (UV) or electron beam (EB) irradiation; and wherein at least one cooling step is provided in connection with or after the UV or EB treatment step.