This paper studies the poetic forms of Lady Mary Wroth’s Sonnet 103 by focusing on its contrasting structural pattern. The paper applies some concepts of New Formalism and New Criticism that emphasize “an organic unity of form and meaning” (Ryan 4) in literary works, especially in poetry. The organic unity of form and meaning of Sonnet 103 is accomplished by using “complex tropes and images such as metaphor， symbol， irony， and paradox” (Ryan 5). The whole sonnet is based on the contrast between mature love and immature and fanciful young love, and the contrast is mainly formed between the second quatrain and the third quatrain. While the second quatrain solely focuses on the merits or “eternal goodness” of true mature love, the third quatrain mainly concentrates on young beginners’ love. The contrasts between the “eternal goodness” of “faithful love” and “these fancies” of “young beginners” established in the first 3 quatrains paves the way for the speaker to effectively convince her lover to “leave of” “these fancies” and prove his honor with “Constancy” in the final couplet. In her concluding Sonnet 103, Pamphilia becomes a mature lover. She is no longer helplessly enchanted by youthful love “fancies”; rather her mature mind now focuses on “truth”, “true joy”, “eternal goodness”, “constancy” and “honor.” Between the lines, her last sonnet seems to urge men and the patriarchic society to address the unjust social reality that never considers man’s love inconsistency as a serious moral defect. This shows that Wroth/Pamphilia has really become a strong speaking subject in her final sonnet that effectively concludes the whole sequence.