Hey guys, i just ran into a problem while trying to render smoke in transparency. As you might see in the screenshot, the alpha of the render doesn’t really give out the real nice alpha values as one would expect seeing the smoke preview in the viewport. should i do something with the materials? im kind of confused 
I believe if you change the selection of the “Channels of the image to draw” at the bottom of the Render window you will see what you are looking for.
I believe he’s showing the alpha channel to show there’s no half-transparent smoke, as if it’s straight alpha. Any other settings on material you’re using? Cannot reproduce it with ‘Quick Smoke’ and the settings from the screenshot
I found that having objects with an alpha chanal half-in and half-out of a volumetric object causes problems. If you put the alpha object either entirly inside or entirely outside the smoke object it should render correctly.
ISSUE: NO ALPHA TRANSPARENCY ON FIRE AND SMOKE?
Do not panic, this is not Blender bug, thing just changed.
I was actually panic the other day. I have been experimenting with Blender Smoke and Fire before 2.65 and now I am using 2.67, suddenly I found out that Alpha is no longer showing when rendering Smoke and Fire.
From : http://blendersushi.blogspot.com/2013/03/vfx-vanilla-fire-and-smoke.html?q=smoke
What is happening?
Luckily Thomas Dinge gave explanation:
“Make sure to switch from “Sky” to “Transparent” in the Shading panel (Alpha setting). The Alpha pipeline changed in Blender 2.66.”
http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.66/Image_Transparency
scource : http://blendersushi.blogspot.com/2013/03/vfx-vanilla-fire-and-smoke.html?q=smoke
Its finally solved.