The influence of the structure of microstructure fiber on the spectral broadening of femtosecond laser pulses is experimentally studied. These experiments demonstrate that the spectral broadening of 35-fs pulses of 820-nm Ti∶sapphire laser radiation transmitted through 2-μm core diameter microstructure fibers can be enhanced by a factor of about 1.25 by increasing the air-filling fraction of the fiber cladding from 60% up to 80%, and ultrabroad band continua which are almost 700 nm extending from 520 to 1200 nm and 1000 nm extending from 350 to 1320 nm are respectively generated from these two kinds of fibers. The comparison shows that the efficiency of the nonlinear spectral broadening process can be noticeably improved by increasing the air-filling fraction of the microstructure fiber cladding, due to a higher degree of light localization in the core of such a microstructure fiber.