Ultralong Si3N4 nanowires (NWs) were successfully synthesized with size controlled in N2 gas by using an efficient method. The diameters of the Si3N4 NWs increased when the flow rate of N2 gas increased, with average diameters of 290 nm from flow rates of 100 ml/min, 343 nm from flow rates of 200 ml/min and 425 nm from flow rates of 400 ml/min. Young's modulus was found to rely strongly on the diameters of the Si3N4 NWs, decreasing from approximately 526.0 GPa to 321.9 GPa; as the diameters increased from 360 nm to 960 nm. These findings provide a promising method for tailoring these mechanical properties of the NWs in a controlled manner over a wide range of Young's modulus values. Vapour-liquid-solid (VLS) mechanisms were used to model the growth of Si3N4 NWs on the inner wall of an alumina crucible and on the surface of the powder mixture. Alumina may be an effective mediator of NW growth that plays an important role in controlling the concentrations of Si-containing reactants to support the growth of NWs on the inner wall of the alumina crucible. This approach offers a valuable means for preparing ultralong Si3N4 NWs doped with Al with unique properties.