The subthreshold characteristic of ultra-thin (i.e. quantum-wire), ultra-short double-gate transistors (symmetric structures) working in the ballistic regime has been analytically modeled. This model takes into account short-channel effects, quantization effects and source-to-drain tunneling (WKB approximation) in the expression of the subthreshold drain current. Important device parameters, such as I off -current or subthreshold swing, can be easily evaluated through this full analytical approach, which also provides a complete set of equations for developing equivalent-circuit model used in ICs simulation.