We demonstrate in this work that the performance of self-assembled monolayer (SAM) molecular devices can be modulated by the composition and supramolecular structure of the molecular layer using mixed self-assembled monolayer strategy. We prepared the mixed monolayer on gold surface (with ca. 1 nm roughness) by co-adsorption of 11-(ferrocenyl)- undecanethiol (FUT, rectifier) and 1-undecanethiol (C11-SH, diluent). Micrometer scale molecular junctions were formed by using indium gallium eutectic alloy (EGaIn) as the top electrode. Electrical characterization of the junction found that the ratio of FUT and C11-SH molecules can tune the rectifying performance of the monolayer device: the higher the proportion of ferrocene is, the better the rectifying performance is. To our surprise, mixed monolayer prepared by 20% C11-SH and 80% FUT mixed solution exhibited higher rectification ratio than pure FUT monolayer, due to reduced leaking current. Surface reflective IR spectroscopy and the monolayer thickness characterization by the ellipsometer revealed loosely packed molecules on the surface in the pure FUT monolayer due to the bulky head group of the FUT and the rough gold substrate. FUT that partially lied down on the surface, or buried in the layer therefore created defects, which in turn become the origin of the leakage current. Upon insertion of C11-SH molecules in between the ferrocene molecules, the molecules in the monolayer become more ordered with the support of the C11-SH, as evidenced by decreased wave number of the C—H stretching mode of methylene group by reflective IR spectroscopy. Meanwhile, an increase in thickness for 80% FUT monolayer relative to pure FUT monolayer implied a better orientation of the FUT molecule in mixed monolayer. The ordered structure and better orientation largely improved the stability and reproducibility of the molecular device, reduced the leaking current and afforded higher rectification ratio. Our approach therefore provides a facile and effective strategy for regulating the performance of monolayer devices by molecule aggregation state.