For a Calabi-Yau 3-fold X, I will give an explict 
computation of Donaldson-Thomas type invariant counting 
pairs (F, V), where F is a zero-dimensional coherent sheaf 
on X and V\subset F is a two dimensional linear subspace, 
satisfying a certain stability condition. This is a rank two 
version of the DT-invariant of rank one, computed by Li,
Behrend-Fantechi and Levine-Pandharipande. I use the 
wall-crossing formula of DT-invariants established by 
Joyce-Song, Kontsevich-Soibelman.