Non-centrosymmetric　organics　are　promising　nonlinear　optical　(NLO)　candidates,　which　always　exhibit　a　short　UV　absorption　cutoff　edge,　and　a　strong　second　harmonic　generation　(SHG)　response,　but　an　overlarge　birefringence.　Here,　in　order　to　optimize　the　birefringence,　the　methyl　modulation　of　pi-conjugated　organic　planar　units　is　proposed　and　implemented　with　the　urea　structure　as　a　template.　Thus,　N-methylurea　and　N,N＇-dimethylurea　crystals　are　obtained　by　partially　replacing　hydrogen　atoms　with　methyl　groups.　This　replacement　reduces　hydrogen　donors　and　weakens　interchain　hydrogen　bonds,　which　facilitates　the　decreasing　density　and　the　parallel　arrangement　of　pi-conjugated　planar　units.　Hence,　both　N-methylurea　and　N,N＇-dimethylurea　crystals　exhibit　not　only　an　optimized　birefringence　(N-methylurea　approximate　to　0.099　and　N,N＇-dimethylurea　approximate　to　0.072　at　546　nm)　but　also　an　enhanced　SHG　response　(N-methylurea　approximate　to　1.2　x　beta-BaB2O4　and　N,N＇-dimethylurea　approximate　to　1.9　x　beta-BaB2O4),　while　maintaining　a　short　UV　absorption　cutoff　edge.　Therefore,　this　work　provides　a　novel　strategy　for　the　structural　design　and　performance　modulation　of　organic　NLO　crystals.　With　the　methyl　modulation　of　urea　structure,　the　birefringence　optimization　is　achieved　in　N,N＇-dimethylurea　crystals,　which　exhibit　an　optimized　birefringence　of　0.072　at　546　nm,　an　enhanced　second　harmonic　generation　response　of　1.9　x　beta-BaB2O4,　while　maintaining　a　short　UV　absorption　cutoff　edge　of　204　nm.　image