The ant colony optimization algorithm was used to search for a feasible power ascension path for a boiling water reactor. The control action of start-up operation includes control rod movements and core flow rate variations. During the process, the core status should satisfy some constraints such as thermal limits, preconditioning interior operating management recommendation guideline, and power flow operation domain. In addition, the control objective is to reach full power as soon as possible and approach the target control rod pattern as close as possible. The results show that the developed method can obtain the satisfactory power ascension path, which is also comparable to the operation record.