Bias affects the true intervention effect in randomized controlled trials (RCTs), making the results unreliable. We evaluated the risk of bias (ROB) of quasi-RCTs or RCTs reported in the Korean Journal of Anesthesiology (KJA) between 2010 and 2016. Six kinds of bias (selection, performance, detection, attrition, reporting, and other biases) were evaluated by determining low, unclear, or high ROB for eight domains (random sequence generation, allocation concealment, blinding of participants, blinding of personnel, blinding of outcome assessment, incomplete outcome data, selective reporting, and other bias) according to publication year. We identified 296 quasi-RCTs or RCTs. Random sequence generation was performed better than allocation concealment (51.7% vs. 20.9% for the proportion of low ROB, P < 0.001 and P = 0.943 for trend, respectively). Blinding of outcome assessment was superior to blinding of participants and personnel (42.9% vs. 15.5% and 23.0% for the proportion of low ROB, P = 0.026 vs. P = 0.003 and 0.896 for trend, respectively). Handling of incomplete outcome data was performed best with the highest proportion of low ROB (84.8%). Selective reporting had the lowest proportion of low ROB (4.7%). However, the ROB improved year by year (P < 0.001 for trend). Authors and reviewers should consider allocation concealment after random sequence generation, blinding of participants and personnel, and full reporting of results to improve the quality of RCTs submitted hereafter for publication in the KJA.