This study evaluates the effectiveness of monetary policy instruments in controlling inflation in Nigeria using annual time series data from 2000 to 2025. Employing the Autoregressive Distributed Lag (ARDL) model and Error Correction Mechanism (ECM), the research investigates both the short-run and long-run relationships between inflation and core monetary policy instruments: Open Market Operations (OMO), Monetary Policy Rate (MPR), and Liquidity Ratio (LRR). Augmented Dickey-Fuller (ADF) unit root tests confirm that all variables are stationary at first difference, justifying the ARDL framework. The ARDL long-run estimates reveal that LRR have statistically significant impacts on inflation, LRR negatively affects inflation, indicating its effectiveness as a contractionary tool. OMO, and MPR, although theoretically important, are found to be statistically insignificant in the long run. The study recommends that the Central Bank of Nigeria (CBN) prioritize liquidity management through adjustments to the liquidity ratio and maintain cautious expansion of the money supply to mitigate inflationary pressures. Additionally, enhancing transparency and credibility in monetary policy operations, improving the effectiveness of OMO.