Spray freeze granulation drying (SFGD) is a technique to produce granules from spherical slurry droplets by spraying and removing the dispersion medium by freeze-drying. However, since most of the studies on SFGD use water as the dispersion medium, there are few examples of SFGD being applied to nitride powders, which are highly reactive with water. It is necessary to design a slurry in which the raw materials can be dispersed in an organic solvent that can be freeze-dried at temperature and pressure conditions similar to those of water, and to observe and understand the internal structural changes of the droplets during freezing. OCT was used to observe the fast phenomenon of freezing opaque slurries. Therefore, the objective of this study is to design a nitride slurry suitable for the SFGD method based on the observation of the freezing behavior of slurry using OCT, which enables non-destructive, high-speed, and high-resolution observation of the internal structure of opaque samples. In this study, tert-butyl alcohol (TBA), cyclohexane (CH), and their mixtures were used as dispersion media. The solvents were mixed in different ratios (TBA:CH = 20:80, 40:60, 60:40, 80:20 (vol. %)). Slurries with a particle concentration of 10 vol% were prepared by adding silicon nitride powder and a dispersant to these organic solvents. The apparent viscosity of these slurries was measured using a rheometer. The freezing behavior of a droplet of the slurry was observed using OCT while measuring temperature, followed by sublimation, and the cross section of the resulting freeze-dried granule was observed using SEM. For reference, the freezing behavior of the dispersion media alone was also observed by OCT. The results showed that the freezing of TBA progressed with numerous hexagonal platelet crystals growing from the bottom in contact with the brass plate. In contrast, no clear crystal growth was observed in CH, due to the cubic crystal structure of the solid phase, but the entire body shrank as it froze. The freezing rate of these organic compounds was slower than that of water. When the mixture was frozen, only one exothermic peak was observed for TBA:CH=40:60, and the entire mixture froze rapidly and uniformly, while two exothermic peaks were observed for the mixture of the other ratios, indicating that TBA:CH=40:60 should be eutectic composition. The structure freeze-dried material prepared from the slurry with these organic solvents had a cellular structure for TBA:CH=20:80 and a lamellar structure, which is characteristic of eutectic reaction, for TBA:CH=40:60. Consequently, it was found that the freezing behavior depends on the type and mixing ratio of TBA and CH, and that the difference in the freezing behavior causes the characteristic internal structures of the freeze-dried products.