Alginate-(Poly-L-Lysine)-Alginate(APA) microcapsules were prepared by Electrostatic Droplet Generator(EDG) technique and the thickness of microcapsule membrane, which was composed by polyelectrolyte complex, were studied in this paper. The theoretical formula was given for the measurement of membrane thickness of APA microcapsules by element analysis of membrane and calculation. The membrane thickness was 7-10 microns by theoretical calculation. On the other hand, the thickness of membrane was measured by SEM and optical microscopy and the results were 7 microns and 12 microns, respectively. The results showed that theoretical calculation is in good accordance with experimental determoination of mermbrane thickness and the membrane thickness of APA microcapsule is about 7-10 microns. The optical microscopy is an easy way to measure membrane thickness.