Intensity modulated radiation therapy (IMRT) has become a widely accepted and efficient treatment technique for many types of cancers. Patient's specific quality assurance (QA) should be performed with QA devices. Stability and sensitivity tests conducted on the ArcCHECK (AC) 3D diode array were performed.Set-up error test with AC was performed. The set-up position moved to lateral (mm), longitudinal (mm) and rotational (°) were 0.5, 1.0, 2.0 and 3.0, respectively. Sensitivity change test of diode array with AC through 230 days was also performed. Same array calibration data was applied to all measurements of volumetric-modulated arc therapy benchmark test through 230 days. Gamma method (2 mm/2% criteria) was performed to analyze the result of all measurements.In the results of positional error, gamma pass rate become degenerate according to positional error became larger. With 0.5 mm or 0.5° positional error, decreasing rate of the pass rate of lateral, longitudinal and rotational were 1.0%, 2.5% and 4.2%, respectively. In the sensitivity change test, the gamma pass rate decreased 2.2%/100 days with same calibration data.AC has highly sensitivity against positional error. Sensitivity of AC has been changed and pass rate was decreased 2.2%/100 days through 230 days. Array calibration should be performed in consideration of change of sensitivity.