A very simple and quick measurement of thermal neutron flux distribution was demonstrated in Kyoto University Critical Assembly (KUCA) in the measurement time of only 10 min with position resolution of about 1mm by using a plastic fiber whose end was painted with scintillator. It is important to obtain the accurate information on neutron flux distribution in critical assembly or research reactor in the fields of reactor physics and some neutron applicational researches.Such informations are desired to obtain as quickly as possible in experimental work or preferably with real time measurement.Presently the distribution measurement is usually done with gold wire activation method(1),which generally takes several hours for irradiation,cooling,cutting the wire into pieces and radiation measurement.Fission chambers(2),aeroball systems(3) etc.are also used,although their position resolutions are poor. The authors tried to utilize an optical plastic fiber with 2mm diameter and 2m length whose end was painted with a mixture of ZnS(Ag) scintillator,LiF and adhesive paste in an weight ratio of about 1:1:2 having a thickness of about 0.3mm and the other end was viewed with a small photomultiplier. Figure 1 shows the configuration of the critical assembly with narrow measuring holes A and B.The fiber was inserted into the holes and moved with a speed of 2.5mm/s by a stepping motor and the output count was recorded in a multi-scaler.