Abstract: In this work some diagnostic techniques are introduced,including emission spectroscopy,laser induced fluorescence,and electrostatic probe.These measurements can be broadly divided into two categories:intrusive and non intrusive.The measurements of electronic temperature,heavy particle,and plume velocity are described and some of the results are analysed.It is emphasized that different diagnostic techniques will produce different results of temperature measurement and the relationships of temperature to techniques are given.It can be seen that laser techniques can investigate the detail of flow properties and will become the main technique for the measurement of Arcjets’ properties.