SIGMA, JURNAL SAINS DAN TEKNOLOGI, Vol. 13, No. 1 Januari 2010, ABSTRAK : An event iterative prediktor for water rocket filigt was developed using MATLAB. This proposed prediktor can be used to solve the problem of phase II as well as strutz performend on spereadsheet. It uses variation of spesific mass as the consequence of the compression. Using the initial temperature are 20 derajad C and 25 derajad C in shootinger model, it needs 0.06 sec to finish the phase Ii whice enhances accurary of the prediction is 20 %. It also shows that the temperature bdecrease when the fluid in water rocket expanding.