diff --git a/src/decimal.jl b/src/decimal.jl
index 3778b4b..1cc05c2 100644
--- a/src/decimal.jl
+++ b/src/decimal.jl
@@ -3,6 +3,18 @@ Base.signbit(x::Decimal) = x.s
 Base.zero(::Type{Decimal}) = Decimal(false, 0, 0)
 Base.one(::Type{Decimal}) = Decimal(false, 1, 0)
 
+function Base.isone(x::Decimal)
+    if signbit(x)
+        return false
+    end
+
+    if x.q < 0
+        c, m = cancelfactor(x.c, Val(10), -x.q)
+        return isone(c) && (m == -x.q)
+    else
+        return isone(x.c) && iszero(x.q)
+    end
+end
 Base.iszero(x::Decimal) = iszero(x.c)
 Base.isfinite(x::Decimal) = true
 Base.isnan(x::Decimal) = false
diff --git a/test/test_decimal.jl b/test/test_decimal.jl
index f514ede..b233af7 100644
--- a/test/test_decimal.jl
+++ b/test/test_decimal.jl
@@ -46,7 +46,14 @@ end
 
     @test isone(one(Decimal))
     @test isone(Decimal(0, 1, 0))
+    @test !isone(Decimal(1, 1, 0))
     @test !isone(Decimal(0, 0, 1))
+    @test !isone(Decimal(0, 1, 1))
+    @test !isone(Decimal(0, 2, 0))
+
+    @test isone(Decimal(0, 1000, -3))
+    @test !isone(Decimal(0, 1000, -2))
+    @test !isone(Decimal(0, 1000, -4))
 
     @test isfinite(Decimal(0, 1, 1))
     @test !isnan(Decimal(0, 1, 1))