I would like to present my recent study on collapsars and formation of jet with the General Relativistic MHD (GRMHD) code that I have developed(ApJ, accepted:arXiv:0902.1908). Also, I have developed a General Relativistic Force-Free (GRFFE) code by which Blandford-Znajek''s paraboloidal solution is reproduced well. I am going to discuss how important it is to develop a hybrid code that includes GRMHD and GRFFE codes toward understanding the central engine of a long GRB. Further, I have done numerical simulations on collapsars using ZEUS code by adding some microphysics such as a realistic equation of state and neutrino cooling/heating (Nagataki et al. ApJ 2007). Finally, I would like to talk about explosive nucleosynthesis in a collapsar (Nagataki et al. ApJ 2006), which can give a strict constraint on the central engine of a long GRB.