In the present work, we have extended some previously calculated bulk-based results of MCE to the scale of NPs, where the catalytic processes actually happen. Atomistic spin dynamics have been performed to simulate the magnetic properties of Ni and Co spherical NPs. We first verified that the simulated 𝑇 ! (∞) for Ni and Co are 631 K and 1380 K, respectively, which accords well to the experiments. Then, we would like to investigate the size-dependence of Curie point, which indicates that 𝑇 ! (𝑑) can be quite lower than its bulk value. As shown in Figure , the relationship between the Curie point and the particle size can be described as Equation , where the fitted parameters are 𝑑 # = 0.33 nm, while 𝜈 = 0.92 for Ni and 𝜈 = 0.84 for Co. These values are consistent with those reported in literature . As a result, the 𝑇 ! of a 1nm NP is dropped by about 25% for Ni and Co. This indicates that the operating temperature of MCE can be greatly reduced if the simulated 𝑇 ! can be directly used to represent the catalytic active sites of the NP catalysts. Then, we have determined the magnetic status of the surface atoms at the NPs to demonstrate further the size-dependence of MCE, rather than the Curie point itself. As shown in Figure , the thermal demagnetization curves of the outer-most shell and the inner core in a 1 nm Ni NP have been separately simulated. It's not surprising that the effective magnetization of the atoms at the outer-most shell is quite lower than the atoms at the inner core due to the atomistic bond loss on the surface . It seems like the surface atoms at the 1 nm Ni NP are more magnetically-disordered, and thus can quickly decrease into the PM phase. Noteworthy is that a finite magnetization above 𝑇 ! can still be observed at the NP (Figure ). Thus, the decrement in 𝑇 ! cannot be directly adopted as direct evidence for the possibility of inducing MCE at a temperature lower than its bulk Curie point. As shown in Figure , a short-range spin polarization still exists at the particle's surface, where the system size is close to the magnetic correlation length . Since the catalytic reactions happen at localized active centers, it seems that the full-range MCE can never occur at NP with a considerably small size.