Abstract: Bamboo stem’s cross sectional area is never a perfect circle, but almost ellipse. Each ellipse shape has a unique value of eccentricity (e) as parameter to denote its circularity. A perfect circle has a zero value of eccentricity. Conventional calculation for bamboo flexural properties as designated by ISO 22157-1:2004 resulted an overestimated or underestimated value compared to the actual value because of the perfect circle cross sectional assumption. Inappropriate geometrical assumption of cross sectional area derived inaccurate value of moment of inertia hereafter affected to the measured flexural properties. Thirty six bamboo stems from 4 species namely Ampel (Bambusa vulgaris Schrad.), Tali (Gigantochloa apus (Bl.Ex Schult.f) Kurz), Gombong (Gigantochloa verticillata (Willd.) Munro) and Mayan (Gigantochloa robusta Kurz.) were harvested and it was found that the eccentricity (e) value of bamboo stem could vary from 0.000 to 0.508. This paper studied the effect of eccentricity to the flexural properties of bamboo and aimed to create the strength ratio (Ce) between actual elliptical shape and assumed perfect circle shape. It was reported that the conventional calculation arise an under estimate result if the major axis arranged horizontally, while overestimate result will be get if the major axis arranged vertically. So the modulus of rupture (SR) which is calculated by conventional calculation should be adjusted by the strength ratio of eccentricity (Ce) in order to define more precise value. This study result the exact relationship between Ce value and eccentricity for both conditions. For simplicity, the graphical sketches were made too.