The depletion of fossil fuels makes it necessary to find alternative fuels to diesel. Waste cooking oil biodiesel (WCO) has interesting properties and could replace diesel fuel. However, WCO biodiesel increased nitrogen oxides (NOx) at high loads compared to diesel. Thus, some fuel additives, such as functionalized MWCNTs and decanol, can be used as they are considered promising diesel fuel additives and significantly affect engine performance and exhaust emission levels. The functionalized MWCNTs, mainly Nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) and amino-functionalized multi-walled carbon nanotubes (NH2-MWCNTs), are excellent catalysts that have been used recently in many applications and have good properties that can be used in diesel engines. Thus, in the current research, an experimental investigation was performed by a CI engine working at a constant speed and different loads to study the effects of a WCO biodiesel-decanol blend on engine performance and exhaust gas emissions. First, n-Decanol was added to WCO biodiesel to enhance the biodiesel combustion properties (80 % biodiesel + 20 % n-decanol by volume). Then, two doses (50 and 75 ppm) for each type of functionalized MWCNTs were inserted into the WCO biodiesel-decanol blend to enhance the engine performance. The results illustrated that WCO biodiesel-decanol blend was close to diesel in engine performance, where the NOx and soot were reduced by about 8 % and 20 % for WCO biodiesel-decanol blends compared to diesel. Also, the addition of functionalized MWCNTs led to a significant reduction in the NOx, CO, and soot levels by around 35 %, 61 %, and 44 % compared to diesel. BTE was diminished by almost 10 %, and BSFC was raised by 15 % for WCO biodiesel-decanol blend compared to diesel. At the same time, these negative impacts have been alleviated with inserting functionalized MWCNTs into the WCO biodiesel-decanol combination. At 75 % load, WCO biodiesel-decanol blends raised the in-cylinder pressure and advanced the start of combustion by a three-crank angle degree. Therefore, it can be implied that WCODNH75 is a recommended combination considering combustion and emission characteristics.