Abstract: Direct charge radioisotope battery having the advantages of long service lifetime, simple structure, high open circuit voltage and easily miniaturized, is a promising source for the great power of Micro-ElectroMechanical System. Although the beta sources are the main choice for direct charge nuclear battery in the present studies,but no systematic analysis and comparison on beta sources is presented. In this work, the properties of six beta sources (including ³H(Ti³H₂), ¹⁴C, ³⁵S, ⁴⁵Ca, ⁶³Ni and ¹⁴⁷Pm) were studied by software simulation and theoretical calculation. This study includes the differences of energy conversion efficiency and the theoretical output power for direct charge nuclear battery. The calculated results showed that the energy conversion efficiency was positively correlated with the average energy of emitted radioactive particles, and the theoretical output power was negatively correlated with half-life of beta source. ¹⁴⁷Pm was the preferred choice considering long life, the energy conversion efficiency and the theoretical output power.