Non-inductive startup by a 2.45 GHz microwave through electron cyclotron resonance heating (ECRH) has been studied in the SUNIST spherical tokamak (R/a: 0.3 m/0.23 m; BT0: < 0.15 T). Two types of discharges, with quite different transient characteristics, were found. The transient processes of discharges are experimentally investigated by scanning the radial position of the ECR layer, the vertical magnetic field and the microwave power. Time domain analysis of microwave reflections and Hα emissions reveal a process dominated by the combination of ionization, loss along the open field lines and the gradient B drift. It is shown that the transient character is insensitive to the experimental parameters except the pressure of filled gas. This result confirms that the control of gas puffing is of great importance for starting up a spherical tokamak by ECRH using low-frequency microwaves.