Objective To identify pulmonary thromboembolism (PTE) using the thrombus attenuation value on unenhanced computed tomography (CT).Design Single-center retrospective study (January 2015-Marcg 2020).Methods Patients who underwent both unenhanced and enhanced CT for suspected PTE were enrolled. Patients with a hyperdense lumen on unenhanced CT and thrombi in the peripheral pulmonary artery (PA) were excluded. Patients were classified into two groups: thrombi (thrombi detected in PA by enhanced CT) and non-thrombi (attenuations of the main PA evaluated as thrombi). Mean CT attenuation values of the thrombi, main PA, and pulmonary trunk (blood pool) were measured. The attenuation values of the thrombus (T) and the thrombus to blood-pool (T/P), thrombus to hemoglobin (T/Hb), and thrombus to hematocrit (T/Ht) ratios were evaluated. The cut-off attenuation value of the thrombus was calculated by a receiver operating characteristic curve and its accuracy in detecting PTE was determined.Results Of the 260 patients enrolled, 40 were included, of whom 24 had confirmed PTE. The mean T was 27.25 Hounsfield units(HU) and 36.66HU (p<0.001), and the T/P ratio was 0.74 and 0.99 (p=0.004) in the PTE and non-PTE groups, respectively. The thrombus cut-off value for PTE diagnosis was 30.85 HU. The sensitivity and specificity were 79. 9% and 87.5%.Conclusions Measuring and evaluating the attenuation value for the central PA and T/P ratio on unenhanced CT improves the diagnostic ability of central PTE in patients suspected to have PTE but cannot tolerate contrast medium.