Femtosecond　(fs)　laser　is　proved　a　powerful　tool　in　the　field　of　3-dimensional　internal　micromachining　inside　the　transparent　dielectrics.　There　are　already　many　types　of　femtosecond　laser　systems　widely　applied　nowadays,　among　which　Ti:　sapphire　femtosecond　laser　and　femtosecond　fiber　laser　are　the　two　most　frequently　used　typical　femtosecond　laser　systems.　However,　according　to　our　study,　the　differences　in　the　laser　parameters　between　these　two　femtosecond　laser　systems　may　induce　significant　discrepancy　in　manufacturing　internal　hollow　structures　inside　the　polymethyl　methacrylate　(PMMA)　substrate.　The　experimental　results　show　that　when　a　65fs　laser　beam　is　focused　inside　the　PMMA　substrate　and　scans　a　route　of　a　straight　line　at　an　average　power　of　1.5W　and　1　kHz　repetition　rate,　a　hollow　microchannel　is　successfully　fabricated　and　no　melted　region　is　found　around　the　microchannel.　However,　if　the　PMMA　substrate　is　constantly　irradiated　with　the　1.5W　laser　pulses　that　generated　by　a　400fs　femtosecond　fiber　laser　system　at　a　repetition　rate　of　100　kHz,　a　growing　hollow　cavity　is　observed　and　a　melted-resolidified　region　is　formed　around　the　cavity.　According　to　the　numerical　simulation　of　heat　accumulation　effect,　we　explain　the　＇heat　accumulation　effect＇　caused　in　the　femtosecond　fiber　laser　manufacturing　and　the　＇cold　processing　property＇　of　the　Ti:　sapphire　femtosecond　laser　processing,　respectively.　These　experimental　and　numerical　results　may　broaden　the　understanding　of　thermal　effect　during　the　process　of　the　femtosecond　laser　micromachining　and　provides　more　opportunities　in　the　manufacturing　of　polymeric　integrated　microfluidic　chip　with　laser　direct　writing　technology　in　the　future.