We have carried out an extensive survey on the HST Halpha images to prepare a catalogue of all ionized bubbles in the disk of the nearby spiral galaxy M81. A visual examination of the 29 HST/ACS fields, that were required to cover the entire M81 disk, resulted in around 300 bubbles of diameter larger than 3 pc. The size distribution of these bubbles peaks at 10 pc, with a tail extending to even larger than 50 pc. We used 10.4-m GTC to carry out follow-up spectroscopic observations of more than 100 of these ionized bubbles in order to classify spectroscopically their ionizing stars. As expected, large bubbles are mostly powered by the Wolf-Rayet (W-R) stars, while none of the bubbles smaller than 20 pc
harbour these massive stars. Early B-type stars are found to be responsible for the ionization of the bulk of the bubble populations in M81. We use our results for M81 to discuss the relative contributions of B-stars as compared to the O-stars to the overall dynamics of the ISM.