In a continuation of our work on the constituents of Algerian plants particularly of Euphorbia species, the present study was carried out on the roots of Euphorbia guyoniana Boiss. & Reut. We describe the isolation and structural determination of two new abietatriene-type diterpenes 1 and 2, together with a known cycloartane-type triterpene 3 (3-hydroxycycloart-24-one). Structures of the isolated compounds were established mainly by 1D and 2D homo- and heteronuclear NMR and ESI mass spectrometry, and by comparison with literature data. We isolated from the chloroform extract of the roots of Euphorbia guyoniana two new ent-abietatriene-type diterpenes 1 and 2 along with one known cycloartane-type triterpene. The known triterpene 3 was identified as 3-hydroxycycloart-24-one by comparison of its NMR spectra and value of [α]D with published data. Compound 1 was elucidated to be 11,16-epoxy-ent-abieta-8,11,15-triene-13,14-dione. Compound 2, with a molecular formula C19H26O2, was analyzed via spectral data including UV, IR, NMR and MS, and its structure was determined based on correlations from 2D NMR experiments and comparison with related compounds.