||Radio frequency identification (RFID) is a rapidly developing technology which uses radio frequency signals for automatic identification of objects. In this paper, a novel design of active RFID tag on a flexible PET substrate is introduced, which is light weight, small size and bendable. This tag includes an organic photovoltaic, a recharge lithium batteries and a gas sensor, in order to utilize sun energy and provide continuous gas monitoring/warning function. Better than other active RF tag based on printed circuit board structure, this tag is small in size, light in weight, and flexible in shape. It is environmentally friendly, as it uses renewable energy as power source and does not need to replace or deposit the battery. The technology of flexible thin film components are studied and modified due to the design requirements. The materials and geometric structures were selected and optimized based on theoretical analysis, finite element method (FEM) simulation, and experimental test, with considerations of the thermo-mechanical performance of the multilayer structure and electrical performance. Packaging methods were selected to satisfy its criteria including high performance, high reliability, low cost, short producing time, low assembly temperature, and environmental friendliness. The packaging methods are suitable for roll-to-roll printing technology, leading to high throughput and low manufacturing cost per unit. This research provides a systematic analysis of technology selection, material selection, structure design, processing optimization, and overall reliability evaluation for an active RTID on flexile substrate. Not limited to active RFID tags, this method also contributes to other multilayer flexible electronic designs.