A new model describing the valve outline of pennate diatom, arc-constitutive model is here advocated. We measured the outline of 30 valves belonging to ten Navicula species using an arc-gauge, and found out that the outline can be resolved into arcs. Dividing the valve by apical- and transapical axis into four quadrant, the outline in each quadrant can be resolved in maxiMurfno u r 'arcs; main arc from the transapical axis, terminal arc from the apical axis, and two or less supplemental arcs smoothly connecting main- and polar axis. The valves were classified into six types based on the number and convexity/concavity of the supplemental arcs. After averaging the diameters and lengths of each corresponding arc in four quadrants, the valve outline was drawn as a combination of the arcs by using computer aided design (CAD). The drawn outlines were almost agreed with that of the original micrographs. We therefore conclude that the arc-constitutive model well explains valve outline of some pennate diatoms, at least of ten Navicula species evaluated in the present study.