Single-phase BaCoTiFe10O19 (BaCoTi-M) nanoparticles were prepared by a modified sol-gel process, using metallic chlorides as starting materials. The physical chemistry process of BaCoTi-M formation, the interdependences between composition, technological conditions, microstructure, and magnetic properties were studied by X-ray diffraction (XRD), Fourier transform-infrared (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). XRD and FTIR results show that BaCoTi-M nanoparticles formed directly from gamma-Fe2O3, spinel ferrite, and barium salts without the formation of alpha-Fe2O3 and BaFe2O4. The lattice shrinkage of BaCoTi-M nanoparticles that occurred on increasing the calcining temperature from 973 to 1173 K under holding for 2 h or on increasing the holding time in the range 0-2 h at 1173 K was discovered by analyzing the dependences of lattice parameters on the heat-treatment conditions. The shrinkage led to a relatively higher concentration of magnetic Fe3+ cations in the unit cell, and resulted in an increase of specific saturation magnetization under the corresponding conditions. Microstructural characterization shows that the evolutions of coercivity, remnant magnetization, and squareness ratio depended on the crystal growth and the reduction of structural defect as well as a decrease of grain boundary.