Fabrication method of microfluidic devices consisting of a sheet (1) which is 200 micrometer thick or less and a rigid part (3), both made of thermoplastic polymeric material includes degasification of a polymeric sheet of thermoplastic material (1), an auxiliary rigid part (2), and a polymeric rigid part of thermoplastic material (3). A temporary bonding procedure, of the degassed thermoplastic polymeric sheet (1) to a degassed auxiliary rigid part (2), is conducted producing a sheet-auxiliary part set (4). A permanent bonding procedure bonds the thermoplastic polymeric sheet (1) of the sheet-auxiliary part set (4) obtained in the previous temporary bonding stage, to the thermoplastic polymeric rigid part (3) which was initially degassed. The auxiliary rigid part (2) of the thermoplastic polymeric sheet (1) permanently bonded to the thermoplastic rigid part (3), is detached to produce a final part which is completely polymeric (5).