The objective of this work was to investigate the existence of polymorphs and pseudopolymorphs of naproxen and the transformation of crystal forms. Four crystal forms of naproxen have been isolated by recrystallization and characterized by differential scanning calorimetry, powder X-ray diffractometry and thermogravimetric analysis. The differential scanning calorimetry and powder X-ray diffractometry patterns of the four crystal forms were different respectively. In the dissolution studies in pH $6.8{pm}0.05$ buffer equilibrated at $37{pm}0.5^{circ}C$, the solubility of four crystal forms was similar (within the error range). After storage of 1 month at 0% RH (silica gel, $20^{circ}C$), 52% RH (saturated solution of $Na_2Cr_2O_{7.}2H_2O/20^{circ}C$) and 95% RH (saturated solution of $Na_2HPO_4/20^{circ}C$), Form 2 and Form 4 were transformed to Form 1, but Form 3 and Form 1 were not transformed and they were shown to have a good physical stability at room temperature for 1 month.