Pyroelectric infrared detectors based on La-modified PbTiO3(PLT) thin films have been fabricated by RF magnetron sputtering and micromachining technology. The detectors form PB1−xLaxTi1−x/4O3(x = 0.05) thin film ferroelectric capacitors epitaxially grown in-situ by RF magnetron sputtering on Pt/ MgO(100) substrate. The sputtered PLT thin film exhibits highly c-axis oriented crystal structure (90%) that poling treatment for sensing applications is not required. The c-axis orientation ratio a of deposited PLT thin film strongly depends on the morphology of Pt layer, which in turn varies with the thickness of Pt layer on MgO substrate. We have successfully grown highly c-axis oriented PLT film on Pt electrode with a conductive percolating network structure. Micromachining technology is used to lower the thermal mass of the detector by coating Polyimide on top of the sensing elements to support the fragile structure and by selectively etching the backside of the MgO substrate to reduce the heat loss. The sensing element exhibited a low noise equivalent power (NEP) of 1.7 × 10−10W and a very high detectivity D* value of 8.5 × 108cmVHz/W at room temperature. The high performance for pyroelectric infrared sensing is primarily due to the highly c-axis oriented PLT thin film and its minimized thermal mass.