High-sensitive pyroelectric infrared sensors have been fabricated with La-modified PbTiO₃(PLT) thin plates. The PLT thin plates have the composition of (Pb_(0.9)La_(0.1)Ti_(0.75)O₃) _(0.75)(PbO)_(0.25). Thickness of the thin plates is 100 ㎛. Top side electrodes exposed to IR are vacuum evaporated Ni-Cr, and bottom side electrodes are Ag. Each one takes the area of 1 x 2 ㎟. The thin plates have a large resistivity of 6.41 x 10 Ω·cm and a relative dielectric constant of 341. They have a high figure of merit of 4.0 x 10^(-11) Ccm/J due to its high pyroelectric coefficient of 4.45 x l0^(-8)C/㎠K. The sensors show such a large voltage responsivity as 2501 V/W. That they can find practical applications like the pyroelectric infrared detectors.