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Hazel (<i>Corylus avellana</i>) is easily attacked by <i>Curculio nucum</i> L. To better understand the physiological mechanisms underlying the different resistance of cultivars to <i>C. nucum</i>, we determined the insect-resistant compounds, plant hormones contents, and enzyme activities in the nutshells of three hazel cultivars (DW, B21, and MZ) before (preexisting defense) and after (induced defense) <i>C. nucum</i> chewing. The findings demonstrated that the resistance of three hazel cultivars to <i>C. nucum</i> differed significantly (<i>p</i> < 0.05): the damage rate of MZ with 17.57% was highest, followed by DW (11.23%), and then B21 (7.15%). The contents of insect-resistant compounds (total terpenoid, tannin, total phenol, flavonoids, cellulose, and lignin) varied with hazel cultivars, both before and after <i>C. nucum</i> chewing, except for cellulose and lignin before induction. The level of plant hormones and defense enzyme activities of hazelnut enhanced due to <i>C. nucum</i> induction. Pearson correlation results revealed that the hazelnut damage rate was significantly negatively correlated with jasmonic acid (JA) (<i>R</i>2 = 0.812), SOD (<i>R</i>2 = 0.671), salicylic acid (SA) (<i>R</i>2 = 0.878), and terpenoids (<i>R</i>2 = 0.774), and significantly positively correlated with flavonoids (<i>R</i>2 = 0.696), celluloses (<i>R</i>2 = 0.501), POD (<i>R</i>2 = 0.758), and abscisic acid (ABA) (<i>R</i>2 = 0.978). The hazelnut defense to <i>C. nucum</i> was negatively related to cellulose contents, and not to lignin contents, but was significantly positively related to the ratio of cellulose-to-lignin (<i>R</i>2 = 0.703). Our results suggested that the hazel against <i>C. nucum</i> attack responded by improving plant hormones contents and enzyme activities in the nutshells. A particular cellulose-to-lignin ratio provides the most effective physical structural defense properties in the nutshells.