Effect of the different annealing conditions on the I-V characteristics of AlGaN with high Al fraction P-I-N photodiodes has been investigated.The result shows,by using an optimised annealing process,not only a good ohmic contact between AlGaN and metal electrode has been obtained,but also the reverse current of the AlGaN P-I-N diode was reduced apparently. At -5V bias,the dark current density and series resistance of diode decreased from 2.0×10-1A/cm2 to 5.7×10-5A/cm2 and from 18.01kΩ to 1.071kΩ individually. I-V characteristic of AlGaN P-I-N device shows improved. It is believed the improvement relates with optimizing of contact properties,elimination of damage induced by device technics and reduction of defect density during annealing process.