Abstract: The physico-chemical compatibility of solid-state pharmaceuticals is mainly de-termined by their interaction. The "Three-point-test" of differential scanning calorimetry in whichthree samples of different ratios are determined is proposed to study this interaction, and the simplifiedjudging rules are as follows: If there are two or less than two endotherm peaks in DSC thermogram for all 3 ratio samples, wemay mainly consider the peaks at lower temperature. If these temperatures are the same, which wouldbe monotectic system or simple eutectic mixture, there will be very weak interaction in their solidstate, and these could be compatible. If the temperatures are different, which would be solid solutionsystem or solid complex system, there would be interaction in their solid state. If there is no endotherm peak, or there are exotherm peaks or there are more than two endothermpeaks, they all indicate that there is a stronger interaction in their solid state and they are incompati-ble. As an example, the interaction between water-soluble vitamins-Vit B₁, Vit B₂, Vit C andnicotinamide was determined. The results indicated that only Vit C is incompatible with nicotinamide,as there is a stronger interaction, and with Vit B₁,there is a weak interaction.