Organic soil causes major problems in infrastructure development. It has high compressibility and low shear strength, and requires chemical stabilization if it is to be a sustainable geomaterial. This research investigated the strength and microstructural properties of organic soil stabilized with magnesium chloride (MgCl2). Unconfined compressive strength tests were undertaken to assess shear strength properties, and microstructural changes were monitored via field-emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectrometry (EDAX). The results confirm that MgCl2 improves the compressive strength of organic soil. The strength of MgCl2-stabilized organic soil is increased to around 3 – 5 times that of untreated soil during the first 7 days of curing. FESEM results show that the porosity of organic soil is filled by a new cementitious compound, identified as magnesium silicate hydrate (M-S-H).