This is a two-year longitudinal study investigating the optical and structural effects of increased compression factor of orthokeratology lens on eyes and the corresponding effect on change in choroidal thickness and therefore myopic control.

Inclusion Criteria:

• 6 to 10 years old
• Myopia: between 0.50 D and 4.00 D in both eyes
• Astigmatism: <1.50 D; ≤ 1.25 D for with-the-rule astigmatism (axes 180 ± 30); ≤ 0.50 D for astigmatism of other axes in both eyes
• Anisometropia: ≤ 1.50 D
• Symmetrical corneal topography with corneal toricity <2.00 D in both eyes
• Agree for randomization

Exclusion Criteria:

• Contraindications for orthokeratology wear (e.g. limbus-to-limbus corneal cylinder and dislocated corneal apex)
• Any type of strabismus or amblyopia
• Myopic treatment (e.g. refractive surgery and progressive lens wear for myopic control) before and during the study period
• Rigid contact lenses (including orthokeratology lenses) experience
• Systemic condition which might affect refractive development (for example, Down syndrome, Marfan's syndrome)
• Ocular conditions which might affect the refractive error (for example, cataract, ptosis)
• Poor compliance for lens wear or follow-up