Aging is associated with the degeneration of several tissues, and that degeneration has an impact on the structure and function of organs. Aging is the most potential risk factor for chronic diseases, particularly dementia. Mitochondrial function is one of the important processes to maintain the metabolism in cells and organs. Mitochondrial function in peripheral blood mononuclear cells (PBMCs) can be a biomarker for several chronic disease. However, the association between aging and mitochondrial function from PBMCs of healthy adult population has not been elusive. Thus, the present study aims to explore the association between aging and mitochondrial function in human PBMCs. The participants were recruited from Electricity Generating Authority of Thailand (EGAT). The demographic data were collected. The basic laboratory parameters were obtained by venipuncture. The PBMCs were also isolated from the venous blood of the participant and then underwent a mitochondrial function test by Seahorse assay. The mitochondrial oxidative stress and mass were assessed by MitoSox and MitoTracker fluorescent dyes and were quantified by flow cytometry. The data of 1,753 participants remained in the final analysis. The associations of age and mitochondrial respiratory parameters were assessed by linear regression analysis. The covariates including underlying diseases of participants and the basic laboratory parameters obtained in this study were used to evaluate the independent associations of age on mitochondrial functions. The general characteristics of the participants in this study were shown in Table 1. The increased age associated with the reduction in mitochondrial ATP production (beta coefficient [95% confident interval] = -0.59 [-0.76,-0.43]) and mitochondrial spare respiratory capacity (-3.30 [-3.65,-2.95]) in PBMCs. Furthermore, the higher age is associated with the higher mitochondrial oxidative stress/mass ratio (0.0045 [0.0040,0.0050]). After adjusted with underlying diseases or basic laboratory parameters, these associations were still in the same direction as univariate analysis. Aging was associated with mitochondrial dysfunction in PBMCs as indicated by the reduction in mitochondrial ATP production and mitochondrial spare respiratory capacity and elevation of mitochondrial oxidative stress/mass ratio. Further investigation on these findings might be useful to develop novel disease interventions or biomarkers for chronic diseases, particularly dementia.