  We present the result of a search for orphan Gamma-Ray Burst (GRB) afterglows
in the Pan-STARRS1 (PS1) COSMOS survey. There is extensive theoretical and
observational evidence suggesting that GRBs are collimated jets; the direct
observation of orphan GRB afterglows would further support this model. An
optimal survey strategy is designed by coupling the PS1 survey with the
Subaru/Hyper-Suprime-Cam (HSC) survey. The PS1 COSMOS survey, one of the survey
fields in the PS1 Medium Deep Survey (PS1/MDS), searches a field of 7 deg$^2$
from December 2011 to January 2014, reaching a limiting magnitude R $\sim$ 23.
The dense cadence of PS1/MDS is crucial for identifying transients, and the
deep magnitude reached by the HSC survey (R $\sim$ 26) is important for
evaluating potential GRB hosts. A transient classification method is employed
to select potential orphan GRB afterglow candidates. After a thorough analysis
of the transient and host galaxy properties, we conclude that there are no
candidates in this survey field. The null result implies that the consideration
of jet structures is essential for further orphan GRB afterglow surveys.
