Large clear crystal of LLTGS (TGS doped with L-lysine) with 60 x 27 x 14 (mm) has been grown by cooling an aqueous solution. The concentration of L-lysine was determined by liquid chromatography. The X-ray diffraction shows that the lattice parameters (a, b and c) are larger than those of pure TGS. LLTGS crystal has excellent pyroelectric properties and higher internal bias field than that of LATGS (TGS doped with L-alanine) crystal, and so it is a promising pyroelectric material for infrared detector.