The probability distribution of prompt neutron number from the spontaneous fission of Pu240 has been measured by use of a spherical liquid scintillation detector 60 em in diameter. The experimental distribution can be well represented by a Gaussian distribution with σ=1.08. This result was compared with the experimental results of B. C. Diven, A. Hicks and J. W. Boldeman, and is in agreement with the semi-emperical theoretical distribution given by J. Terrel. The absolute efficiency of the detector for the fission neutron of Pu240 has been found to be 0.759 ± 0.011, based on the average number of prompt neutrons per fission for spontaneous fission of Pu240 v=2.154±0.028[5].