Monitoring the power distribution of reactor core is one of the most challenging issues in the safe operation of nuclear reactors. The Number and arrangement of detectors in a core monitoring system should be determined in a way that maximum amount of information about core states is extracted, while avoiding duplication of similar and correlated measurements. Information theory is the best method to deal with the problem of optimal number and placement of instruments. In this paper, the effects of reactor fuel burn-up on the optimal number and arragement of in-core fixed detectors of Tehran Research Reactor has been investigated using information theory. Comparison the results for the two modes, with and without consideration of the fuel burn-up, shows that by considering fuel burn-up in the placement problem, more information can be extracted from measurements of the detectors. Also, for an equal number of detectors, determined arrangement by considering fuel burn-up, mostly provide larger share of total available information in the data set. Using the optimal arrangement, maximum information can be obtained from minimum number of the detectors, which leads to considerable decrease in the cost of construction, implementation, operation, and maintenance of the in-core detectors for monitoring of neutron flux.