Purpose: In intensity modulated radiation therapy (IMRT) and/or volumetric modulated arc therapy (VMAT), it is important to ensure the accuracy of multi‐leaf collimator (MLC) motion during delivery. However, the commercially available software can evaluate only summary of MLC motion during delivery as the error's root‐mean‐square (RMS). The purposeof this study is to construct the suitable procedure to evaluate MLC‐motion continuously with an in‐house program. Method: This study was composed into two sections. First, we created the in‐house program to import MLC‐log file during delivery. The in‐house program was built by using visual basic language. The in‐house program includes some additional: 1) visualization of MLC motion during delivery with an animation, 2) classification whether the MLC‐motion error was over the tolerance or not, and 3) visualization of the location of MLC‐motion error during delivery. Second, we evaluated the MLC‐log during VMAT delivery by using our in‐house program. The VMAT plan was created by Eclipse ver.8.9 (Varian). The Clinac 21EX linear accelerator (Varian) equipped with Millenium multi‐leaf collimator was used for this study. Results: We confirmed the functions of our in‐house program as follows. The in‐house software was available to import the Varian's formatted MLC‐log file. The animation of MLC motion during delivery was expressed in 2 fr/sec. The tolerance value of MLC motion error could be defined optimally. If the MLC motion error happened, the location became colored from default color to red color. For prostate VMAT, the MLC motion error in all cases passed the tolerance value. We defined the tolerance value based on AAPM TG‐119 and 142. Conclusion:We conclude that our in‐house program should be useful to evaluate the accuracy of MLC motion during IMRT and/or VMAT delivery because the users can analyze the MLC motion error simply. © 2012, American Association of Physicists in Medicine. All rights reserved.