An analysis of spike-like noise for the thermal detector prepared with Pb(Zr,Ti)O3-Pb(Sb0.5Nb0.5) (PZT- PSN) pyroelectric ceramics as sensing element was conducted by measuring its oven noise as a function of the JFET characteristics, gate resistance, low-temperature heat treatment, chemical composition and grain size of pyroelectric ceramic. Pyroelectric wafers were prepared by mixed oxide technique, and thermal sensor fabricated with a PZT-PSN ceramic wafer, JFET, chip-type gate resistor and TO- 5 package with AR coated Si-window. White noises depended on the characteristics of JFET, gate resistance and chemical composition of puyroelectric materials. Output spikes originated in JFET were removed by the pyroelectric sensing wafer with high capacitance. Pyroelectric element generated the temperature-induced transient noise during cooling, which were remarkably reduced in their amplitude and frequency by heat-treating at low temperature and by decreasing the grain size of pyroelectric ceramic. The spike-like transient noise is caused by the twinning and domain switching to reduce the thermally induced elastic energy within the sensing element, originated in the different thermal expansion between pyroelectric ceramic and alumina substrate.