Using petroleum pitch is touted as a sustainable method to fabricate carbon fibers, yet requires further advances in inevitable pore decrease after post-treatment. In an effort to circumvent this upper limit, renewable resources are widely used in the production of commercial carbon fibers. Here we show that dissolved micro-nanobubbles of CO2 in pretreatment may aid in the pore-growth of carbon fibers during activation. The results confirm that micro-nanobubbles increase specific surface (39.21 %) and micropore (16.44 %) areas of a sample. The chemical state of the elements revealed that there were no marked impurities. The observed behaviors can be understood by the following; 1) Partial O atoms released from dissolved micro-nanobubbles may attach to surrounding CO during activation, thereafter results in a higher mass of CO2. 2) Partial O atoms may directly interact with C from unmatured crystallites and form additional CO. We further denote optimized conditions based on the derived mechanism. This study provides a new strategy for the development of highly surface carbonaceous materials, thus possibly stimulating more research on advanced performance adsorbents or electrode materials.