NaYF4:Yb, Er/NaYF4 core-shell nanocrystals with different thickness shells were synthesized. The correlation between shell thickness and upconversion (UC) luminescence intensity was investigated experimentally and theoretically. We found that the UC fluorescence intensity of the core-shell nanocrystals is enhanced exponentially with shell thickness (d) in the form of (1-0.9 exp(-d/d0). For our core-shell nanocrystals, the d0 was determined as about 5.5 nm, corresponding to an enhancement of about 12 times for the 540 nm emission intensity. The d0 may be treated as the optimized shell thickness, which represents a balance between the conflict requirements of strong UC fluorescence intensity and small total crystal size for bioapplications.