One-dimensional angular correlation of annihilation radiation (1D-ACAR) and positron lifetime in crystalline KDP (potassium dihydrogen phosphate) are measured in the temperature range between 100K and 295K. Positronium, formation in the crystal is confirmed by magnetic quenching of positronium. The momentum distribution of the positronium component in the 1D-ACAR spectrum has a width of about 4 X 10(-3)mc in FWHM showing that the positronium is not in a Bloch state. Intensity of the narrower component in ACAR spectrum is largest at the ferroelectric transition temperature (123K). This implies that positron annihilation characteristics in KDP are correlated with the ferroelectric phase transition.