A new kind of long-chain aliphatic polyamide (PA1218) with a relatively low melting point, high molecular weight, and stable mechanical properties at humid conditions was successfully developed via a polycondensation reaction between 1,18-octadecanedioic acid and 1,12-diaminodecane. Additionally, oleic acid-surfaced modified silicon dioxide (SSD) was prepared and employed to improve the properties of PA1218 through in-situ polymerization. FT-IR spectra and TGA thermograms confirmed the successful surface modification of nanoparticles, and consequently, 5% substitution of surface hydroxyl groups of SiO₂ nanoparticles with oleic acid molecules. Moreover, the thermomechanical and rheology tests revealed a significant improvement in nanocomposites’ properties compared to the pure PA1218; for instance, the tensile strength and storage modulus were increased by 22% and 40%, respectively in the sample containing 3% SSD nanoparticles. This improvement, along with SEM images, confirmed the uniform dispersion of SSD nanoparticles through the employed in-situ polymerization and excellent compatibility between inorganic and organic phases, which was achieved via surface modification. Finally, all the samples demonstrated a water uptake capacity of less than 0.6% attributed to the high methylene/amide ratio in their backbones, causing these newly developed nanocomposites to be notable candidates for specific engineering applications.