Neutron flux spatial oscillation due to xenon build-up in the reactor core is a well-known characteristic of large power reactors. Especially a reactor with a positive temperature coefficient of reactivity tends to have such a characteristic. In this paper, the analysis has been pursued on a Calder Hall type reactor. Specific features to be taken into consideration were a large neutron flux flattened zone in the core and a graphite sleeve in each coolant channel. First, the threshold values of the temperature coefficient for initiating oscillation of successive orders of modes in radial and azimuthal directions as well as oscillation periods have been calculated. Secondly, the effect of the sleeve on threshold value and oscillation period has been investigated. Thirdly, in order to clarify this phenomenon, a vector analysis has been made which helps us to understand the critical condition for initiating the oscillation as well as the relation between effects of neutron leakage, temperature coefficients of fuel and moderator, and xenon poisoning. Finally, taking advantage of the transfer function defined in each mode, the spatial control method could be analyzed without using a spatial simulator.