Zhu, D.C. Cao, J.S. Sui, Y.F. Wang, J.Q. Yue, J.H. Zhang, W. JACoW Publishing Geneva, Switzerland 2673-5350 978-3-95450-230-1 10.18429/JACoW-IBIC2021-WEPP01 English 353-355 impedance synchrotron electron storage-ring radiation Contribution to a conference proceedings 2021 2021-10 https://doi.org/10.18429/JACoW-IBIC2021-WEPP01 https://jacow.org/ibic2021/papers/wepp01.pdf We measure the bunch length at BEPC II electron storage ring using a dual sweep streak camera at visible light diagnostic beamline. The impedances estimated by a series R+L impedance model. Resistive impedance of R=446±21 Ω is obtained by measuring loss factor from measured synchronous phase advancing with streak camera. An inductance impedance of L=23.3±1.8 nH has been estimated by measuring single bunch lengthening with beam current. Both loss factor and inductance are close to the impedance budget. Besides, the streak cam-era is also used to measure synchronous phase at low current as RF voltage changing from 0.85 MV to 1.65 MV.