A new method to measure compressibility of a vesicle suspension was developed with optical tweezers. By compressibility measurement, we can study about interactions between vesicles. Large unilamellar vesicles were prepared with DOPG, cholesterol and DiI. In the ”optical bottle” created by the IR laser, we observed concentration increase near the beam spot. Using concentration increase versus IR laser intensity results, we developed the method to estimate the trapping energy per particle and virial coefficients. We also calculated compressibility of DOPG vesicle suspension and compared these results with theoretical ideal gas and hard-sphere cases.