The central bank is a sovereign institution that plays a pivotal and active role in the banking and financial system, therefore the related studies have special importance. This study aims to clear the concept of profitability and asset and liability management at the central banks because of the mysterious which surround them, especially in light of the general objectives that the central banks pursue to achieve. The growing interest in the subject of profitability due to its role in strengthening the financial position of the central bank and strengthen its ability to carry out its functions effectively, as well as to indicate the sources of income of the central banks, and identify the strategies followed by the central banks to manage its assets and liabilities, and their effect on profitability. In order to achieve these objectives, the researcher, after the presentation of the theoretical framework for the study, conduct an analytical study of the financial statements contained in the published reports of the Reserve Bank of New Zealand, during the period from 1/7/2005 to 30/6/2011, in order to clarify the theoretical framework for the study, and its compatibility with practice. Thus, the researcher conducting statistical test to show the effect of changes in the size of central bank assets and liabilities on a net income of the Central bank, has been using the statistical program SPSS 18 to reach the results of this test. The study concluded that the changes in the sizes of the assets and liabilities of the central bank does not affect its net income, in the other words that we can't predict the profitability of central bank depending on the size of its assets or liabilities. The lack of profit maximization as a key target does not mean that Central does not seek through its management of its assets and liabilities for profit, but it means that his main goal is to achieve the objectives of monetary policy and then make a profit. Seiniorage is the most important sources of income of the central and most stable, while the most important function of the Central bank which is formulating monetary policy do not generate any revenue. In addition to that there are many factors that threaten central bank's profitability such as dollarization, electronic money and low inflation levels.