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　π-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　π-conjugated　planar　units.　Hence,　both　N-methylurea　and　N,N＇-dimethylurea　crystals　exhibit　not　only　an　optimized　birefringence　(N-methylurea　≈0.099　and　N,N＇-dimethylurea　≈0.072　at　546 nm)　but　also　an　enhanced　SHG　response　(N-methylurea　≈1.2　×　β-BaB2O4　and　N,N＇-dimethylurea　≈1.9　×　β-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.　©　2024　Wiley-VCH　GmbH.