The time of flight experiment is performed on the hydrogenated amorphous silicon p-i-n diode. External field is varied from 3000 to 20000 V/cm. The drift mobility is measured when complete charge collection is confirmed. The TOF experiment is also performed without external field and the internal field profile of the p-i-n diode is obtained. For comparison, the above procedure is repeated in the condition of AM1 illumination. For the dark condition, measured drift mobility is 0.046 ㎠/V.sec and the depletion layer width is 4700 Å. For the AM1 illumination condition, the drift mobility measured is 0.036 ㎠/V.sec and the depletion layer width is 3900 Å. The potential profile of the p-i-n diode is also calculated using determined field profile for both conditions and the results are discussed.