Hi Qinghua,
Basically you need to make a transparent color, then use it to color the ribbon.

Please see this previous post for details:
<http://plato.cgl.ucsf.edu/pipermail/chimera-users/2006-December/001155.html>

Best,
Elaine
----------
Elaine C. Meng, Ph.D. 
UCSF Computer Graphics Lab (Chimera team) and Babbitt Lab
Department of Pharmaceutical Chemistry
University of California, San Francisco



On Sep 28, 2010, at 11:28 PM, Qinghua Liao wrote:

Dear chimera's users,
Could someone tell me how to set the transparency of ribbon of a protein? Thanks very much 
All the best,
Qinghua