Delays in budget execution till the end of the fiscal year have long been of major concern to governments worldwide. In an attempt to deal with this, the Taiwanese government introduced a midyear budget execution review into its budgetary system. This study aims to examine the impact of this extra monitoring mechanism on budget execution patterns. Using the defense units as a sample, we find that the first half-year budget execution rate increases and the high concentration of budget execution for the second half of the year is significantly reduced after the introduction of the midyear review. Overall, the results indicate that public budget execution suffers from managers' opportunism and that closer monitoring helps mitigate such opportunistic behavior.