Two novel soluble organic n-type semi-conductors (PDI-C20 and PDI-Gly-C20) have been successfully synthesized. Their π-Stack formation was characterized by NMR, X-ray diffraction and FT-IR. The equilibrium constants K between free and π-stacked molecules, which were deduced from NMR, exhibit a stronger tendency of forming π-stacks for PDI-Gly-C20 than PDI-C20. The X-ray diffraction results also suggest a significantly shorter intra-stack separation between two adjacent molecules in PDI-Gly-C20. Stronger π-stacking interaction in PDI-Gly-C20 will lead to enhanced charge transport performance, which makes it a promising material in the field of optoelectronics.