There has been significant work investigating the use of self assembled monolayers (SAM) made of ferrocenyl terminated alkanethiols for realizing molecular diodes leading to remarkably large forward-to-reverse current rectification ratios. In this study, we use a multi band barrier tunneling model to examine the electrical properties of SAM-based molecular diodes made of HSC 9 Fc, HSC 11 Fc and HSC i FcC 13−i (0 ≤ i ≤ 13). Using our simple physical model, we reproduce the experimental data of charge transport across various ferrocenyl substituted alkanethiols performed by Nijhuis, Reus and Whitesides (J. AM. CHEM. SOC. 2010, 132, 18386-184016) and L. Yuan and al. (Nature Communication 2015, 6:6324). Especially, the model allows predicting the rectification direction in HSC i FcC 13−i (0 ≤ i ≤ 13) based molecular diodes depending on the position of the ferrocenyl (Fc) moiety within the molecules. We show that the asymmetry of barrier length at both sides of the Highest Occupied Molecular Orbital (HOMO) of the ferrocenyl moiety strongly contributes to the rectifying properties of ferrocenyl-alkanethiol based molecular junctions. Furthermore, our results reveal that bound and quasi-bound states play an important role in the charge transport.