Aim To investigate the mechanisms of hypertensive microalbuminuria and the benefit of early intervention with losartan. Methods The investigation comprised normotensive control (WKY) and hypertensive group (SHR) of rats, of which blood pressure, microalbuminuria, renal function and relevant ultrastructural indexes in vessel and glomerulus with transmission electronic microscope were acquired at three month old and eight month old. Results In three month old SHR, electronic microscopic observation demonstrated early impairment of vessel endothelium and slight lesion of glomerular basement membrane compared with age matched controls. As a result, microalbuminuria that correlated with blood pressure and impairment of endothelium has been shown. At eight month old, creatinine level has not elevated in spite of the aggravation of impairment of vessel endothelium and lesion of glomerulus, which was associated with microalbuminuria. However, microalbuminuria was unrelated to blood pressure at this stage. With the treatment of losartan, not only reduction of microalbuminuria was achieved but also decrease in volume density of vessel endothelium, mesangial cells and mesangial area was shown. Conclusions The mechanism led to microalbuminuria in hypertension may vary according to the severity of the hypertensive condition. Early intervention with losartan can inhibit glomerulosclerosis more than lower blood pressure.