Cucumber plug seedlings were cultured in a phytotron to study their responses to different daytime temperature. The daytime temperature was controlled at 30 degrees C, 27 degrees C, 24 degrees C, 21 degrees C, 18 degrees C and 15 degrees C, and the night temperature was at 15 degrees C, with the thickness and length of hypocotyl, length and width of the first and the second leaves, dry matter accumulation of above- and underground parts, leaf moisture content, and leaf chlorophyll fluorescence parameters measured. Principal component analysis and clustering analysis were made to analyze the seedling quality under different temperature treatment. There existed significant differences in the test growth indices among different treatments, with the seedling quality decreased at the daytime temperature 24 degrees C > 21 degrees C > 27 degrees C > 30 degrees C > 18 degrees C > 15 degrees C. All the daytime/nighttime temperature treatments could be classified into three groups, i.e., optimum temperature (24 degrees C/15 degrees C), appropriate temperature (21 degrees C/15 degrees C), and inappropriate temperature, and the inappropriate temperature could be further subdivided into two groups, i.e., high-temperature inhibition (27 degrees C/15 degrees C, 30 degrees C/15 degrees C) and low-temperature inhibition (15 degrees C/15 degrees C, 18 degrees C/15 degrees C).