【Aim】 The comparative ultrastructure of the achenes in six similar species in the genus Lactuca was studied for their micro-morphological identification and to provide a reference basis for the classification ranking below a genus and above a species. 【Method】 Scanning electron microscopy (SEM) was used to observe micro-morphological characteristics of achenes from six similar species, including surface character, pappus, hilum and collar-shaped ring. The length, width of spikes and length between spikes and pappi were measured, and one-way analysis of variance was used to determine the significant differences for the analysis of difference. 【Result】 The hilum of L. pulchella had no annular edge, while the other five species had a rounded spikes on achene. Surface of L. pulchella. and ear-shaped spikes on that of L. tatarica were observed. Collar-like rings in L. tatarica and L. sativa were round or nearly round, while the other four speices are ellipse. The length of the pappus of L. tatarica and L. hirsuta was the smallest, and that of L. serriola was the longest, followed by L. pulchella, L. sativa and L. india. Burr bases of the pappus in all six species except for L. serriola were not enlarged. The length, width of spikes and length between spikes and pappi indicated significant difference among the six species. 【Conclusion】 The characters, such as the shape of hilum, the presence or absence of the ring-shaped edge, the central convexity of hilum, collar-like ring, surface texture and outrigger, can be used as the basis for identification of the genus Lactuca. The significant differences in the length, width of spikes and length between spikes and pappi segment can be used to identify the genus Lactuca, which has certain significance for exploring the distinction between species and the relationship analysis between groups.