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Almost any type of gravity inversion modeling suffers from the problem of non-uniqueness, especially in time-lapse microgravity problem. The uncertainty can not be completely reduced because in general the inverse problem of geophysics is usually non-linear, so using linear algorithm can not find optimal solution. In this research, we present non-linear inversion using a very fast simulated annealing method to find out the spread of density changes made in the reservoir. Our technique has been applied to synthetic data and proven can be identify the location of density contrast at the reservoir.