Neutron generators (NG) are being increasingly used in various industrial and research areas such as neutron activation analysis, neutron radiography, and neutron capture therapy. In such applications of neutron generators, compactness is one of the most important issues. Since a neutron source is generated by deuterium-deuterium (D-D) or deuterium-tritium (D-T) fusion reaction, a relatively thick shield for both fast neutrons and related photons is usually required. In this study, optimization of shielding designs for D-D and D-T neutron generators were investigated by adopting appropriate moderator and shielding materials. Based on the optimized moderator and shield thicknesses, the final dimensions of neutron generators were derived for various source strengths of D-D and D-T neutron conditions. Considering conventional condition of a container, we concluded that a 1010 n/sec D-D source and a 108 n/sec D-T source could be a portable NG.