Accidental and dispersion of toxic gases were always major hazards for public health and safety that process industries had to deal with. Real terrain dispersion simulation for risk analysis purpose is needed to evaluate any risk potential. This paper describe possibilities of predicting the model of gas dispersion for the simulation of 3D gas mapping application. Relative mass loss of the liquid state of the gas will be introduced as inputs of the process that will be modeled. Simulation study shows OE (Output Error) model shows promising result in identifying the system. The OE model also show comparable and superior results to ARX (Auto Regression with Extra Input) and AR (Auto Regression) model respectively. © 2009 IEEE.