Recent　progresses　in　femtosecond　laser　(fs)　manufacturing　have　already　proved　that　fs　laser　is　a　powerful　tool　in　three　dimensional　internal　structure　fabrications.　However,　most　studies　are　mainly　focused　on　realize　such　structures　in　inorganic　transparent　dielectric,　such　as　photosensitive　glass　and　fused　silica,　etc.　In　this　study,　we　present　two　methods　to　fabricate　embedded　internal　3D　structures　in　a　polymer　dielectric　material　polymethyl　methacrylate　(PMMA).　Both　continuous　hollow　structure　such　as　microfluidic　channels　and　discrete　hollow　structures　such　as　single　microcavities　are　successfully　fabricated　with　the　help　of　femtosecond　lasers.　Among　them,　complicated　3D　microchannel　with　a　total　length　longer　than　10mm　and　diameters　around　80　mu　m　to　200　mu　m　are　fabricated　with　a　low　repetition　rate　Ti:　sapphire　femtosecond　laser　by　direct　laser　writing　at　a　speed　ranging　from　25　mu　m/s　to　2000　mu　m/s;　microcavities　which　function　as　concave　microball　lenses　(CMBLs)　and　can　be　applied　in　super-wide-angle　imaging　are　fabricated　with　a　high　repetition　rate　femtosecond　fiber　laser　due　to　the　distinct　heat　accumulation　effect　after　5s　irradiation　with　the　tightly　focused　fs　laser　beam.　These　new　approaches　proved　that　femtosecond　laser　direct　writing　technology　has　great　application　potential　in　3D　integrated　devices　manufacturing　in　the　future.