Abstract: From the extra pressure of curved liquid surface, state equation of ideal gas and the related equations of spheroid, the connecting big and small soap bubbles is studied and the approximate equation of the radius of the final big bubble is deduced. The demonstration experiment is introduced, three different pictures of the three bubbles are taken, and the radii are measured. The approximate equation is verified and the reason of error is analyzed

Key words: surface tension, soap bubble, extra pressure, diffusion