We present a photometric study of the stars in ionizing star clusters embedded in several giant HII regions of M33 ( CC93 , IC 142 , NGC 595 , MA2 , NGC 604 and NGC 588 ) .
Our photometry are based on the HST-WFPC2 images of these clusters .
Color-magnitude diagrams and color-color diagrams of these clusters are obtained and are used for estimating the reddenings and ages of the clusters .
The luminosity functions ( LFs ) and initial mass functions ( IMFs ) of the massive stars in these clusters are also derived .
The slopes of the IMFs range from \Gamma = -0.5 to -2.1 .
It is found interestingly that the IMFs get steeper with increasing galactocentric distance and with decreasing [ O/H ] abundance .