In this study we examine quantitatively the correlation between the geometric lip contour parameters and the formant parameters for Mandarin monophthongs, and carry out a multiple linear regression study between the two parameters. We explicitly analyze the relationship between different geometric lip parameters and the formant parameters, which have some linguistic significance instead of the usual acoustic parameters such as MFCC. We analyzed the linguistic meaning of the regression formula, and found it in accord with the classical result on the relationship between vocaltrack and speech acoustics. And those regions with relatively poor effect of estimation are related to specific phonetic conditions.