We propose a framework to study the optimal liquidation strategy in a limit order book for large-tick stocks, with the spread equal to one tick. All order book events (market orders, limit orders and cancellations) occur according to independent Poisson processes, with parameters depending on the most recent price move direction. Our goal is to maximise the expected terminal wealth of an agent who needs to liquidate her positions within a fixed time horizon. By assuming that the agent trades (through both limit and market orders) only when the price moves, we model her liquidation procedure as a semi-Markov decision process, and compute the semi-Markov kernel using Laplace method in the language of queueing theory. The optimal liquidation policy is then solved by dynamic programming, and illustrated numerically.Date: November 16, 2017.2010 Mathematics Subject Classification. 91G60, 91G99, 60K15, 60K20. Key words and phrases. limit order book, optimal liquidation, semi-Markov decision process, queueing theory, dynamic programming.The authors would like to thank Martin Gould and Fabrizio Lillo for useful discussions. AJ acknowledges financial support from the EPSRC First Grant EP/M008436/1 and HL acknowledges financial support from SHELL. . 1 A priority rule regulates how limit orders stored in the LOB will get executed. By far the most common priority rule is 'price-time' [24, Section 3.4], that is, limit orders posted closer to the mid price will get priority and limit orders posted at the same price follow the 'first come first serve' rule.