Contamination of groundwater by petroleum-hydrocarbons is a widespread environmental problem. Because the petroleum-hydrocarbon resulted plumes could be quite diffuse and widespread, some more economic approaches are desirable for groundwater remediation to provide for long-term control of contaminated groundwater. Monitored natural attenuation (MNA) has been considered as a passive remedial approach to degrade and dissipate contaminants in groundwater. In this study, a full-scale and detailed natural bioremediation investigation was conducted at a petroleum-hydrocarbon spill site in Kaohsiung County, Taiwan. In this natural attenuation study, the following tasks were conducted: (1) groundwater analysis; (2) evaluation of the occurrence of natural attenuation, (3) calculation of biodegradation capacity and natural attenuation rate calculation, (4) evaluation of the percent loss of hydrocarbons due to biodegradation processes by BIOSCREEN model, and (5) application of BIOPLUME III model for the development of remedial strategies. Results show that benzene, toluene, ethylbenzene, and xylene isomers (BTEX) concentrations dropped to below detection limit (BDL) before they reached the downgradient monitor well located 280 m from the spill location. A first-order decay model was applied for the natural attenuation rate calculation. Results reveal that natural biodegradation process was the major cause of the BTEX reduction among the natural attenuation mechanisms. Results from the groundwater analyses indicate that mixed anaerobic biodegradation patterns occurred between the source and mid-plume area, and the aerobic biodegradation dominated the mid and downgradient area. Approximately 74% of the BTEX removal was due to intrinsic biodegradation processes. The calculated natural attenuation rates for BTEX, methyl tert-butyl ether (MTBE), and 1,2,4-trimethylbenzene (1,2,4-TMB) were 0.13, 0.06, and 0.19 1/day, respectively. Evidence for the occurrence of natural attenuation was the decreased contaminant mass flux through the plume cross-sections along the transport path. Evidences for the occurrence of natural BTEX biodegradation included the following: (1) depletion of dissolved oxygen (DO) within the plume; (2) production of biodegradation by-products [Fe(II), CO2, and methane] within the plume; and (3) decreased BTEX concentrations and BTEX as carbon to TOC ratio along the transport path. The calculated biodegradation capacity (45 mg/L) at this site is much higher than the detected concentrations of petroleum-hydrocarbons (1.5 mg/L) within the most contaminated area inside the plume. Thus, natural biodegradation should be able to remove the contaminants effectively. Results suggest that natural attenuation mechanisms can effectively contain the plume and cause the significant removal of petroleum hydrocarbons. Moreover, pump-and-treat and air sparging systems are also feasible technologies to remediate contaminated groundwater at this site.