This paper describes an experimental study of the enlargement of the vertical beam size observed in single beam operation at the KEKB LER. 1 INTRODUCTION A blow-up of the vertical beam size is observed in the KEKB positron ring (LER). The beam size as a function of beam current starts to increase at a threshold beam current and is almost doubled at 300 mA under typical operating conditions. Thus the blow-up is one of the most serious problems limiting the luminosity of KEKB. The main characteristics of the blow-up observed in early operation period are summarized as: 1) the blow-up is a single beam and a multi-bunch effect; 2) the blow-up has a threshold intensity which is determined roughly by (bunch current)/(bunch spacing); 3) no dipole oscillation has been observed when the vertical chromaticity is high (5 to 8), though the chromaticity is as low as 2 and the bunch feedback system is turned off, a vertical dipole oscillation is excited; 4) the blow-up is almost independent of ..