your problem is that even if blending is enabled pixels are always written to the buffers (color/depth). even completely transparent ones. if you want to disable writing of transparent pixels you should enable alpha testing. another solution (but this depends what you want to achieve) is to disable writing to depthbuffer completely (glDepthMask(GL_FALSE) or disable depth buffer testing. this way elements won't be 'sorted' but with this example it doesn't really matter.