The　unit　cell　method　is　widely　used　in　thermal　calculations　of　composite.　The　formulation　of　a　unit　cell　involves　construction　of　geometric　configuration　and　derivation　of　boundary　conditions.　The　process　is　closely　related　to　the　structure　symmetries　of　composites　and　the　macro　heat　flux.　In　this　work,　two　issues　are　studied　to　further　appropriate　utilization　of　such　method.　First,　unit　cells　that　formulated　based　on　different　paths　with　different　structure　symmetries　could　have　the　same　configuration　while　different　boundary　conditions,　and　leads　to　some　confusions;　second,　the　uniform　temperature　boundary　condition　is　frequently　used　without　rigorous　mathematical　derivation　and　physical　consideration,　and　leaves　some　uncertainty　of　its　reliability.　In　this　work,　three　typical　composites,　unidirectional　fiber　reinforced,　plain　woven　and　three-dimensional　four-directional　braided　composites　are　studied.　For　each　composite,　one　unit　cell　is　formulated　and　two　types　of　boundary　conditions　are　derived.　The　influence　of　formulation　path　on　unit　cell　formulation　and　the　application　scope　of　uniform　temperature　boundary　conditions　are　then　clarified　based　on　corresponding　calculations　and　analysis.