diff --git a/Tests/PhotochkTests/UnionFindTests.swift b/Tests/PhotochkTests/UnionFindTests.swift
new file mode 100644
index 0000000..9cb96be
--- /dev/null
+++ b/Tests/PhotochkTests/UnionFindTests.swift
@@ -0,0 +1,33 @@
+import Testing
+
+@testable import Photochk
+
+@Suite("UnionFind")
+struct UnionFindTests {
+    @Test("結合していない要素はそれぞれ単独の成分になる")
+    func isolatedElements() {
+        var uf = UnionFind(count: 4)
+        let components = uf.components().map { $0.sorted() }.sorted { $0[0] < $1[0] }
+        #expect(components == [[0], [1], [2], [3]])
+    }
+
+    @Test("推移的に結合した要素は1つの成分にまとまる")
+    func transitiveMerge() {
+        var uf = UnionFind(count: 5)
+        uf.union(0, 1)
+        uf.union(1, 2)
+        uf.union(3, 4)
+
+        let components = uf.components().map { $0.sorted() }.sorted { $0[0] < $1[0] }
+        #expect(components == [[0, 1, 2], [3, 4]])
+    }
+
+    @Test("同じ成分をもう一度 union しても何も変わらない")
+    func repeatedUnion() {
+        var uf = UnionFind(count: 3)
+        #expect(uf.union(0, 1) == true)
+        #expect(uf.union(1, 0) == false)
+        #expect(uf.find(0) == uf.find(1))
+        #expect(uf.find(0) != uf.find(2))
+    }
+}