In this study, peptides with six different compositions were designed and synthesized to study the effect of the structure of the hydrophobic block in triblock oligopeptide amphiphiles ((A(m)X)(n)H5K15) on self-assembly properties, drug loading capacity and gene expression efficiency. The peptides were synthesized using a standard Fmoc-solid phase peptide synthesis protocol, which are A(12)H(5)K(15), A(11)FH(5)K(15), (A(5)F)(2)H5K15, (A(3)F)(3)H5K15, (AF)(6)H5K15 and (AL)(6)H5K15. A(12)H(5)K(15), A(11)H(5)K(15), (A(5)F)(2)H5K15 and (A(3)F)(3)H5K15, formed micelles at concentrations above 200 mg . L-1 with large size or aggregation. However, (AF)(6)H5K15 and (AL)(6)H5K15 had CMC values of 42 and 49 mg . L-1 respectively, and the resulting micelles were much smaller in size as compared to the other peptide designs (108 and 233 nm for (AL)(6)H5K15 and (AL)(6)H5K15, respectively). In addition, both peptides were able to load paclitaxel (PTX) into nanoparticles although PTX-loaded (AF)(6)H5K15 nanoparticles had a smaller size (278 versus 295 nm, respectively). Encapsulation efficiency of PTX with (AF)(6)H5K15 nanoparticles was 74%. Gene transfection studies showed that luciferase expression level induced by (AF)(6)H5K15 micelles in HepG2 cell line was much higher than that mediated by (AL)(6)H5K15 micelles at their respective optimal N/P ratios (3.2 x 10(8) versus 3.9 x 10(7) RLU . mg proteins(-1)). Therefore, simultaneous delivery of PTX and luciferase-encoding plasmid was conducted using (AF)(6)H5K15 micelles against HepG2 cells, and the results demonstrated that the co-delivery of PTX at 0.01 mg . L-1 further increased luciferase expression level from 3.2 x 10(8) to 8.0 x 10(8) RLU . mg proteins(-1) at the optimal N/P ratio (i.e., 18). In summary, it is important to optimize the hydrophobic block of the oligopeptide amphiphiles in order to achieve desired properties for co-delivery of drugs and genes.