Abstract: A method for effectively increasing mode field diameter (MFD) of photonic crystal fibers (PCFs) is demonstrated in this paper. Air holes in the cladding of a PCF will be collapse when the PCF is heated due to surface tension. Theoretical and experimental results show that energy loss is very low if the transition connecting the original PCF and the collapsed section is adiabatic. The collapse of air holes can effectively increase the MFD of PCFs. The increase of MFD can not only evidently enhance optical coupling efficiency and surface damage limit, but also apparently reduce splicing loss caused by the mismatch of MFDs with other conventional step fibers.