diff --git a/DIRECTORY.md b/DIRECTORY.md
index 69c7685..1051e25 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -88,6 +88,7 @@
   * [Pascal Triangle Ii](https://github.com/TheAlgorithms/Ruby/blob/master/dynamic_programming/pascal_triangle_ii.rb)
 
 ## Maths
+  * [3N Plus 1 or Collatz Conjecture](https://github.com/TheAlgorithms/Ruby/blob/master/maths/3nPlus1.rb)
   * [Abs](https://github.com/TheAlgorithms/Ruby/blob/master/maths/abs.rb)
   * [Abs Max](https://github.com/TheAlgorithms/Ruby/blob/master/maths/abs_max.rb)
   * [Abs Min](https://github.com/TheAlgorithms/Ruby/blob/master/maths/abs_min.rb)
diff --git a/maths/3nPlus1.rb b/maths/3nPlus1.rb
new file mode 100644
index 0000000..083cdd5
--- /dev/null
+++ b/maths/3nPlus1.rb
@@ -0,0 +1,44 @@
+# A ruby program for 3N plus 1
+# 3N plus 1 is also called as Collatz Conjecture
+#
+# f(n) => n / 2 (if n = 0 (mod 2))
+# f(n) => (3n + 1) (if n = 1 (mod 2))
+#
+# Wiki: https://en.wikipedia.org/wiki/Collatz_conjecture
+
+def collatz_conjecture(number)
+  n = number
+  nums = []
+  nums.push(number)
+
+  while number > 1
+    if number.even?
+      number /= 2
+      nums.push(number)
+    else
+      number = 3 * number + 1
+      nums.push(number)
+    end
+  end
+
+  "The 3N + 1 series of #{n} is #{nums}."
+rescue StandardError
+  'Error: Please provide number only!'
+end
+
+# Valid inputs
+puts collatz_conjecture(12)
+# The 3N + 1 series of 12 is [12, 6, 3, 10, 5, 16, 8, 4, 2, 1].
+
+puts collatz_conjecture(6)
+# The 3N + 1 series of 6 is [6, 3, 10, 5, 16, 8, 4, 2, 1].
+
+puts collatz_conjecture(100)
+# The 3N + 1 series of 100 is [100, 50, 25, 76, 38, 19, 58, 29, 88, 44, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1].
+
+# Invalid inputs
+puts collatz_conjecture('12')
+# Error: Please provide number only!
+
+puts collatz_conjecture('a')
+# Error: Please provide number only!