Hi Haixiao, The Chimera scale bar is positioned in 3 dimensions -- it has a certain depth in the scene. The depth is important because Chimera uses perspective projection by default -- so things closer to you appear larger. So it is important to position the scale bar depth to match the depth of the model or feature you are trying to measure. You can do this by clicking on the "Move scale bar..." switch in the Scale Bar dialog and then using dragging with ctrl button 2 to adjust the depth. It will be helpful to look at the side view (Favorites / Side view) while doing this to see where the scale bar is relative to the model in the depth dimension. Another solution to this problem is to switch to orthographic projection mode. Use Favorites / Side View and switch to the Camera tab and change the "projection" menu value from perspective to orthographic. Then the whole scene is projected perpendicular to the screen. Closer objects will not appear any larger than far objects in that mode. Tom