Photosensitivity has been observed and Bragg gratings have been created in various polymer optical fibres in recent years. Nevertheless the mechanisms of Bragg grating formation in polymer optical fibres are yet to be fully investigated and understood. We carried out experimental investigations on the dynamic growth of Bragg grating under various exposure conditions. For the first time, we observed clearly a threshold behaviour in the grating creation process. The threshold distinguishes two types of fibre gratings that have quite differently properties and performance.