At present, micro-spacecrafts are being developed. Micro-nozzles are very important in propulsion systems of micro-spacecrafts. So further study on micro-nozzles is essential. DSMC(direct simulation Monte-Carlo)calculations for the flow in axisymmetric micro-nozzles are conducted on molecular-based models. The influence of the mass flow rate and dimension on the flow and performance in the nozzle are also discussed. The study indicates that the mass flow rate and dimension of the nozzle have different degrees of influence on the flow and performance.