Niobium aluminum carbide (Nb4AlC3), as a member of the MAX phases, can retain its stiffness and strength up to over 1400°C, however, the strength at room temperature is relatively low. In this work, Nb5(Si, Al)3 was used to strengthen Nb4AlC3. Nb4AlC3–Nb5(Si, Al)3 composites with different amount of Nb5(Si, Al)3 were synthesized from the elemental powders by in situ hot-pressing/solid–liquid reaction synthesis process. The RT flexural strength was significantly improved from 370 MPa of monolithic Nb4AlC3 to 432 MPa of Nb4AlC3-15 vol% Nb5(Si, Al)3 composite without the degradation of its high-temperature mechanical properties. While a slightly decrease in fracture toughness occurs with the increment of Nb5(Si, Al)3 content. Meanwhile, Transmission electron microscopy (TEM) observations reveal that the interfaces between Nb4AlC3/Nb4AlC3, Nb4AlC3/Nb5(Si, Al)3, and Nb5(Si, Al)3/Nb5(Si, Al)3 are free of amorphous layers, which is also beneficial to its high-temperature mechanical properties.