We present an analytical model for the subthreshold characteristic of ultra-thin Independent Double-Gate (IDG) MOSFET working in the ballistic regime. 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 off-state current or subthreshold swing, can be easily evaluated through this full analytical approach. The model can be successfully implemented in a TCAD circuit simulator for the simulation of IDG MOSFET based-circuits.