PLANCK is an ESA scientific mission dedicated to produce accurate map of the temperature fluctuations of the Cosmic Microwave Background, the fossil radiation carrying direct information from the early universe. The launch is foreseen in 2007 by an Ariane 5 rocket, together with the ESA cornerstone mission FIRST. The selected operational orbits for both satellites are Lissajous orbits around the second Lagrangian Libration point (L2) in the Earth - sun system. The PLANCK scientific payload is composed basically of the two scientific instruments HFI (High Frequency Instrument) and LFI (Low Frequency Instrument) which detectors are located at the focal plane of a 1.5 m Gregorian telescope. The LFI detectors are based on HEMT amplifier technology and need to be cooled down to 20 K. The detectors for the HFI are bolometers operating at 0.1 K. These temperature levels are obtained using a set of different coolers, which need pre-cooling stages for normal operation (the coldest one is around 60 K). Finally, the telescope temperature must be lower than 60 K.To meet those requirements, a PLANCK Payload Module accommodating a multi-stages cryogenic passive radiator has been studied in Alcatel Space and Dornier Satellitensysteme. The coldest passive stage offers a ~1.5 W net cooling capacity around 55 K. This high efficiency radiator is based on a large black painted open honeycomb surface radiatively insulated from the warm spacecraft by a set of conical shields opened towards cold space, also called “V-grooves”. Thermal, mechanical, straylight and AIV aspects has been analysed in detail during a one year feasibility study of the PLANCK Payload Module performed in 1999 by Alcatel and Dornier for ESA. The description of the thermal design and an analysis of the main thermal results are presented in this paper.