Highly oriented diamond films were deposited on Si (001) substrates by the microwave plasma assisted chemical vapor deposition technique (MPCVD) using the bias enhanced nucleation (BEN) step. The formation of highly oriented diamond (HOD) films is often linked to the presence of few ppm of nitrogen which stabilizes the texture but deteriorates the chemical quality of the film. In this study, we show that carbon monoxide can be used without nitrogen addition to conserve the <001> texture and to promote the {100} facet extension on the top of the diamond film. After a nucleation step carried out by the BEN technique, and after a subsequent growth time limited to 30 min in a CH 4 /H 2 gas mixture avoiding the coalescence step, HOD films were synthesized using CH 4 /H 2 , CO/H 2 and N 2 /CH 4 /H 2 gas mixtures for a growth time fixed to 10~14 hrs. The effect of carbon monoxide or nitrogen on the morphologies and the texture of the diamond films is discussed from the results of scanning electron microscopy (SEM) and the texture measurements performed by X-ray diffraction.