Nickel oxide nanoparticles decorated carbon nanotube nanocomposites(Ni O·CNT NCs)were prepared in a basic medium by using facile wet-chemical routes. The optical,morphological, and structural properties of Ni O·CNT NCs were characterized using Fourier transformed infra-red(FT-IR), Ultra-violet visible(UV/Vis) spectroscopy, field-emission scanning electron microscopy(FESEM), X-ray energy dispersed spectroscopy(XEDS), X-ray photoelectron spectroscopy(XPS), and powder X-ray diffraction(XRD) methods. Selective4-aminophenol(4-AP) chemical sensor was developed by a flat glassy carbon electrode(GCE, surface area: 0.0316 cm~2) fabricated with a thin-layer of NCs. Electrochemical responses including higher sensitivity, large dynamic range(LDR), limit of detection(LOD), and long-term stability towards 4-AP were obtained using the fabricated chemical sensors. The calibration curve was found linear(R~2= 0.914) over a wide range of 4-AP concentration(0.1 nmol/L–0.1 mol/L). In perspective of slope(2 × 10~(-5)μA/μM), LOD and sensitivity were calculated as 15.0 ± 0.1 pM and ~ 6.33 × 10~(-4)μA/(μM·cm) respectively. The synthesized Ni O·CNT NCs using a wet-chemical method is a significant route for the development of ultrasensitive and selective phenolic sensor based on nano-materials for environmental toxic substances. It is suggested that a pioneer and selective development of 4-AP sensitive sensor using Ni O·CNT NCs by a facile and reliable current vs voltage(I–V)method for the major application of toxic agents in biological, green environmental, and health-care fields in near future. The optical, morphological, and structural properties of Ni O · CNT NCs were characterized using Fourier transformed infra-red (NiO · CNT NCs) were prepared in a basic medium by using facile wet-chemical routes. (FT-IR), Ultra-violet visible (UV / Vis) spectroscopy, field-emission scanning electron microscopy (FESEM), X-ray energy dispersed spectroscopy (XEDS), X-ray photoelectron spectroscopy (XPS) Selective 4-aminophenol (4-AP) chemical sensor was developed by a flat glassy carbon electrode (GCE, surface area: 0.0316 cm ~ 2) fabricated with a thin-layer of NCs. Electrochemical response including higher sensitivity The large dynamic range (LDR), limit of detection (LOD), and long-term stability towards 4-AP were obtained using the fabricated chemical sensors. The calibration curve was found linear (R ~ 2 = 0.914) over a wide range of 4-AP concentration (0.1 nmol / L-0.1 mol / L). In perspective of slo LOD and sensitivity were calculated as 15.0 ± 0.1 pM and ~ 6.33 × 10 -4 μA / (μM · cm) respectively. The synthesized Ni O · CNT NCs using a wet-chemical method is a significant route for the development of ultrasensitive and selective phenolic sensor based on nano-materials for environmental toxic substances. It is suggested that a pioneer and selective development of 4-AP sensitive sensor using Ni O · CNT NCs by a facile and reliable current vs voltage (I-V) method for the major application of toxic agents in biological, green environmental, and health-care fields in near future.