A new kind of bowtie related structures, Sierpinski complementary bowtie aperture (SCBA), is proposed for enhancing and confining optical magnetic field. The magnetic field enhancement factor can be improved with the presence of the fractal. Numerical simulation shows that higher iteration of the SCBA is responsible for the variation of the resonant wavelengths. The magnetic field distributions illustrated 10nm above the output plane prove the magnetic intensity is confined in the sub-wavelength scale. Further investigation demonstrates current can be enhanced in the center of the apertures while electric fields are found to be easier to concentrate at the tiny fractal structures. Magnetic intensity distributions at several resonant points in the spectrum of the third-iteration SCBA are also plotted and the longer wavelength region in near infrared is found to be unsuited for confining magnetic field in SCBA due to weak interaction with the fractals.