The authors have observed diesel spray structures by laser sheet Miescattering methods at room temperature under several ambient pressures(from 0.0 KPa to 2.1 MPa), and measured spray penetrations, spray-anglesand spray inner structures of light diesel oil. The results of spray-anglemeasurements showed that the spray-angle increased with an increase in theambient pressure near a nozzle tip. The spray-angle far from the nozzletip had the minimum value of spray angle at about 0.6 MPa ambientpressure. The spray structure had periodic high density portions occurringat a constant frequency (about 80 KHz). The frequency was time invariantand independent of ambient pressure.These results suggested that theperiodic portions were caused by the inner structure of the injector orthe structure of the nozzle.