In this research, the controllable synthesis of carbon nanotubes (CNTs) functionalized with derivatives of diazonium salts containing -NO2 was carried out under mild and more efficient conditions (i.e. lower reaction temperature, shorter reaction time, assisting microwave and using radical scavenger) to study the morphology of these compounds before and after functionalization. For this purpose, the reactions of single-walled (SWCNT) and multi-walled (MWCNT) carbon nanotubes with 2-nitroaniline, 4-nitroaniline and 2,4-dinitroaniline in the presence of isoamyl nitrite as diazotization agent were performed at different conditions (i.e. solvent free, in dimethylformamide (DMF) solvent, thermal and microwave). The nitroaryl functionalized CNTs were characterized by X-ray diffraction (XRD) measurement, field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, energy dispersive X-ray (EDX) technique and elemental map analysis. The results clearly confirmed the successful controllable functionalization of CNTs by nitroaryl diazonium ions without any byproducts. The morphology of CNTs after functionalization is tubular. Functionalization of SWCNT by nitroaryl diazonium ions improves the crystallinity of starting material; while the crystallinity of MWCNT decreases with functionalization. The samples synthesized under microwave are more amorphous than samples synthesized under thermal conditions.