Applications of graphene have extended into areas of nanobio-technology such as nanobio-medicine, nanobio-sensing, as well as nanoelectronics with biomolecules. These applications involve interactions between proteins, peptides, DNA, RNA etc. and graphene, therefore understanding such molecular interactions is essential. For example, many applications based on using graphene and peptides require peptides to interact with (e.g., noncovalently bind to) graphene at one end, while simultaneously exposing the other end to the surrounding medium (e.g., to detect analytes in solution). To control and characterize peptide behavior on a graphene surface in solution is difficult. Here we successfully probed the molecular interactions between two peptides (cecropin P1 and MSI-78(C1)) and graphene in situ and in real-time using sum frequency generation (SFG) vibrational spectroscopy and molecular dynamics (MD) simulation. We demonstrated that the distribution of various planar (including aromatic (Phe, Trp, Tyr, and His)/amide (Asn and Gln)/Guanidine (Arg)) side -chains and charged hydrophilic (such as Lys) side -chains in a peptide sequence determines the orientation of the peptide adsorbed on a graphene surface. It was found that peptide interactions with graphene depend on the competition between both planar and hydrophilic residues in the peptide. Our results indicated that part of cecropin P1 stands up on graphene due to an unbalanced distribution of planar and hydrophilic residues, whereas MSI-78(C1) lies down on graphene due to an even distribution of Phe residues and hydrophilic residues. With such knowledge, we could rationally design peptides with desired residues to manipulate peptide graphene interactions, which allows peptides to adopt optimized structure and exhibit excellent activity for nanobio-technological applications. This research again demonstrates the power to combine SFG vibrational spectroscopy and MD simulation in studying interfacial biological molecules.