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We present detailed morphological properties of Lyman alpha emitters (LAEs)at z~ 5.7 in the COSMOS field, based on {\it Hubble Space Telescope} AdvancedCamera for Surveys (ACS) data. The ACS imaging in the F814W filter covered 85LAEs of the 119 LAEs identified in the full two square degree field, and 47LAEs of them are detected in the ACS images. Nearly half of them are spatiallyextended with a size larger than 0.15 arcsec (~0.88 kpc at z=5.7) up to 0.4arcsec (~2.5 kpc at z=5.7). The others are nearly unresolved compact objects.Two LAEs show double-component structures, indicating interaction or merging ofbuilding components to form more massive galaxies. By stacking the ACS imagesof all the detected sources, we obtain a Sersic parameter of n~0.7 with ahalf-light radius of 0.13 arcsec (0.76 kpc), suggesting that the majority ofACS detected LAEs have not spheroidal-like but disk-like or irregular lightprofiles. Comparing ACS F814W magnitudes (I_814) with Subaru/Suprime-Cammagnitudes in the NB816, i', and z' bands, we find that the ACS imaging in theF814W band mainly probes UV continuum rather than Lyman alpha line emission. UVcontinuum sizes tend to be larger for LAEs with larger Ly\alpha emissionregions as traced by the NB816 imaging. The non-detection of 38 LAEs in the ACSimages is likely due to the fact that their surface brightness is even too lowboth in the UV continuum and Lyalpha emission. Estimating I_814 for the LAEswith ACS non-detection from the z' and NB816 magnitudes, we find that 16 ofthese are probably LAEs with a size larger than 0.15 arcsec in UV continuum.All these results suggest that our LAE sample contains systematically largerLAEs in UV continuum size than those previously studied at z~6.

HST/ACS Morphology of Lyman Alpha Emitters at Redshift 5.7 in the COSMOS Field