<p>The preconsolidation pressure (σ′_p) plays an important role in analyses of deformation of natural clays. The Red River Delta, the second largest delta in Vietnam, consists of soft to medium stiff clay layers across the delta; however the σ′_p value of the clays has not been investigated systematically. This paper presents an investigation of σ′_p and overconsolidation ratio (OCR) values of clays in the delta. For this, undisturbed clay sampling and cone penetration test (CPTu) were experimentally carried at fourteen test sites across the delta, following by a series of standard consolidation tests in the laboratory. Four methods, i.e., Casagrande (1936), Silva (1970), Butterfield (1979), and Becker et al. (1987) and two CPTu-based equations were applied to obtain the σ′_p value from the consolidation test results and CPTu data. Taking σ′_p value from the Casagrande method as reference one, the correlations from the database show that Silva, Butterfield and Becker et al. methods result in smaller, similar and, larger values, respectively. Comparison of OCR profiles with depth from Butterfield and Becker et al. methods and two CPTu-based equations indicate that the OCR of clays at the investigated sites are normally consolidated (OCR ≈ 1.0) to slightly overconsolidated (OCR = 1.0 to 4.0. Since the investigated sites locate across the delta, this finding may be generally applied to all clays of Holocene period in the delta. Results from this study provides significant insights into the understanding stress history of the clays in the delta.</p>