Background: It is very important to prevent a compromised side branch ostium after deploying a stent that we evaluate the correct side branch ostium length when deploying a stent in the coronary bifurcation lesion. Objectives: We compared the differences between measured and calculated side branch ostium length in the coronary bifurcation lesion with intravascular ultrasound (IVUS). Methods: Between January 2007 and March 2010, 113 patients who underwent IVUS imaging and received stent implantation in the bifurcation lesion were retrospectively analyzed. Side branch ostium length (MSBOL), side branch diameter (SBD), and angle between distal portion of main branch and side branch (AMS) were measured with IVUS, we evaluated the correlation between MSBOL and calculated side branch ostium length (CSBOL) with the trigonometric function: CSBOL = SBD / sin (AMS). Results: Mean age of study patients was 61.3짹9.8 years. The absolute number of male and female patients was 88 and 25. Mean SBD was 2.49짹0.66 mm (pre-stent), 2.44짹0.63 mm (post-stent), and 2.47짹0.65 (all cases). Mean AMS was 57.3짹12.4 degree (pre-stent), 59.4짹12.6 degree (post-stent), and 58.3짹12.5 degree (all cases). Mean MSBOL was 2.91짹0.86 mm (pre-stent), 2.79짹0.82 mm (post-stent), and 2.85짹0.84 mm (all cases). Mean CSBOL was 3.06짹0.77 mm (pre-stent), 2.92짹0.69 mm (post-stent), and 2.99짹0.74 mm (all cases). CSBOL correlated with MSBOL positively: r=0.863, p<0.001 (pre-stent), r=0.868, p<0.001 (post-stent), and r=0.865, p<0.001 (all cases). Conclusion: There were good correlations between MSBOL and CSBOL before and after stent implantation.