Ultrafine $CoFe_{1.9}Bi_{0.1}O_4$ particles were fabricated by using a sol-gel method and their magnetic and structural properties were investigated with an x-ray diffractometer (XRD), a vibrating sample magnetometer (VSM), and a M$"{o}$ssbauer spectrometer. The result of x-ray diffraction and M$"{o}$ssbauer spectroscopy showed that the powders fired at and above 523 K had only cubic spinel structures. M$"{o}$ssbauer spectra measurements showed that the powders annealed at 523,723 and 823 K possessed ferrimagnetic nature and paramagnetic nature due to superparamagnetism, simultaneously at room temperature and the powders annealed at and above 923 K behaved ferrimagnetically. In the case of the powder annealed at 923 K, the lattice constant was $a_0=8.398$pm$0.005{AA}$</TEX> and the hyperfine fields were $H_{hf}(A)=479kOe,; H_{hf}(B)=502kOe$. The isomer shifts indicate that the iron ions are ferric at tetrahedral[A] and octahedral sites [B], respectively. The magnetization as a function of annealing temperature increased as increasing annealing temperature. The largest coercivity values were $H_C=1368;Oe$ AT 923 K annealing temperature. In the case of the powder annealed at 1123 K, the magnetization value was $M_S=75;emu/g$ and this value was similler to that of $CoFe_2O_4$.Fe_2O_4$</TEX>.