We determined the most probable atomistic structure of an ultra-low-k material (k≅ 2.5) by computer simulations. Among the candidate structures generated by a molecular-dynamics calculation, the most probable one that reproduces the observed properties was selected using a first-principles density-functional-theory calculation. The candidate structures consisted of Si-O-Si network with some silicon atoms, each of which had a CH3 group or a hydrogen atom bonded. The structure with CH3 groups but no hydrogen atom reproduced the experimental properties best. This structure was then used to investigate the behaviors of the material irradiated with ultraviolet light.