The correlations among magnetic properties, synthesis temperature, and composition of FeCo nanoparticles were investigated herein. Fe80Co20 alloy nanoparticles synthesized at different temperatures (383, 393, 403, 413, 428, and 443 K) showed variable compositions and aggregation degrees of the FeCo nanoparticles. Under the optimized conditions of synthesis temperature of 403 K and duration of 1 h, FeCo nanoparticles were synthesized at molar ratios of 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, and 1:9. The FeCo alloy nanoparticles were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, vibrating sample magnetometer, fourier transform infrared, and network analyzer. With increasing Co content, the extent of aggregation increased. The cobalt ferrite phase was detected under some conditions, and all FeCo nanoparticles showed high saturation magnetization and low coercive forces. The prepared FeCo nanoparticles exhibited high permeability at a high frequency range.