An experimental investigation was conducted in a closed natural circulation (NC) loop to study the characteristics of pressure drop oscillations (PDO) under low pressure condition. A 3 x 3 rod bundle channel was used as the test section. The experimental results show that PDO can occur in the closed NC loop when the pressurizer is installed on the upstream of the heating channel. The large amplitude thermal-hydraulic parameters oscillations and reverse flow are the main characteristics for PDO. The oscillation amplitude and reverse flow rate of PDO can be enlarged by increasing the heat flux. increasing the inlet subcooling degree can help to suppress the PDO occurrence in the closed NC loop. The flow instability boundary is obtain and PDO mainly occurs in the region of the outlet equilibrium quality from -0.008 to 0.012 in present experiment. The interaction between the vapor expansion in the heating channel and the compressibility of pressurizer is the main reason for the occurrence of PDO in the closed NC loop. The effects of compressible volume positions on flow instability was also studied. Moving the pressurizer from the upstream of the heating channel to the downstream can eliminate PDO while density wave oscillations (DWO) can still happen. A tentative explanation for the influence mechanism of compressible volume positions on flow instability in the closed NC loop is reported. (C) 2018 Elsevier Ltd. All rights reserved.