In order to clarify the unsteady flow fields at low flow-rate region with positive gradient on pressure-flow-rate curve,the experimental investigation was carried out at rotor inlet and outside of rotor blade tip without casing in a semi-opened propeller fan using a hot-wire anemometer.A single I-type hot-wire probe was used,and the data obtained were processed by the use of phase-locked averaging,ensemble averaging and FFT analyzing.The flow fields at rotor inlet and outside of rotor blade tip were discussed mainly using the results from distributions of velocity fluctuations and power spectrum density.It was found from these results that there are the two types of different periodical fluctuations and both of those frequencies were not the same of rotor rotating frequency(RRF;15Hz).One was observed at relatively high flow-rate region at relatively downstream area in measurement and its frequency was approximately 7Hz(47% of RRF).The other was observed at relatively low flow-rate region at relatively upstream area in measurement and its frequency was approximately 10Hz(67% of RRF)“.As the velocity fluctuations with the flow fields are rapidly increased by the former fluctuation,it is thought that its fluctuation is the trigger of blade stall. In order to clarify the unsteady flow fields at low flow-rate region with positive gradient on pressure-flow-rate curve, the experimental investigation was carried out at rotor inlet and outside of rotor blade tip without casing in a semi-opened propeller fan using a hot-wire anemometer. A single I-type hot-wire probe was used, and the data obtained were processed by the use of phase-locked averaging, ensemble averaging and FFT analyzing. flow field at rotor inlet and outside of rotor blade tip were discussed mainly using the results from distributions of velocity fluctuations and power spectrum density. It was found from these results that there are the two types of different periodical fluctuations and both of these same were not the same of rotor rotating frequency (RRF; 15 Hz ) .One was observed at relatively high flow-rate region at relatively downstream area in measurement and its frequency was approximately 7Hz (47% of RRF). The other was observed at relatively low flow-rate regi on the relatively upstream area in measurement and its frequency was approximately 10 Hz (67% of RRF) ”. As the velocity fluctuations with the flow fields are rapidly increased by the former fluctuation, it is thought that its fluctuation is the trigger of blade stall .