In this paper we calculate the escape fraction ( f _ { esc } ) of ionizing photons from starburst galaxies .
Using 2-D axisymmetric hydrodynamic simulations , we study superbubbles created by overlapping supernovae in OB associations .
We calculate the escape fraction of ionizing photons from the center of the disk along different angles through the superbubble and the gas disk .
After convolving with the luminosity function of OB associations , we show that the ionizing photons escape within a cone of \sim 40 ^ { \circ } , consistent with observations of nearby galaxies .
The evolution of the escape fraction with time shows that it falls initially as cold gas is accumulated in a dense shell .
After the shell crosses a few scale heights and fragments , the escape fraction through the polar regions rises again .
The angle-averaged escape fraction can not exceed \sim [ 1 - \cos ( 1 { radian } ) ] = 0.5 from geometrical considerations ( using the emission cone opening angle ) .
We calculate the dependence of the time- and angle-averaged escape fraction on the mid-plane disk gas density ( in the range n _ { 0 } = 0.15 - 50 cm ^ { -3 } ) and the disk scale height ( between z _ { 0 } = 10 - 600 pc ) .
We find that the escape fraction is related to the disk parameters ( the mid-plane disk density and scale height ) roughly so that f _ { esc } ^ { \alpha } n _ { 0 } ^ { 2 } z _ { 0 } ^ { 3 } ( with \alpha \approx 2.2 ) is a constant .
For disks with a given WNM temperature , massive disks have lower escape fraction than low mass galaxies .
For Milky Way ISM parameters , we find f _ { esc } \sim 5 \% , and it increases to \approx 10 \% for a galaxy ten times less massive .
We discuss the possible effects of clumpiness of the ISM on the estimate of the escape fraction and the implications of our results for the reionization of the universe .