An activation method is proposed for in-core neutron flux distribution measurement. A neutron flux distribution being considered is transformed into integral activities by using an activation fluid circulating through a core. Then a set of measured integral activities is transformed inversely to a neutron flux distribution which is approximated by a series of shape functions, such as the Fourier series. Numerical simulation of the results reveal that typical axial neutron flux distributions in a boiling water reactor can be measured within an accuracy of ±15%.