How to rotate an object? I want to rotate object in blender, for which I am using following operations.

ob = bpy.context.active_object
ob.rotation_euler = (x,y,z)


It is working well. Now if reaped the this definition for same object, it doesn't consider the last updated orientation. It considers the original orientation.

What can I do?
You are setting the rotation.

The code in question you are setting the Euler rotation.  The resulting value will always be (x, y, z)

Instead recommend make a transform (rotation) matrix. Python console example. x, y, and z are 30, 45 and 60 degrees respectively.

&gt;&gt;&gt; x, y, z = (radians(30), radians(45), radians(60))
&gt;&gt;&gt; Euler((x, y, z))
Euler((0.5235987901687622, 0.7853981852531433, 1.0471975803375244), 'XYZ')


The context object, has Euler rotation set to zero.

&gt;&gt;&gt; ob = C.object
&gt;&gt;&gt; ob.rotation_euler
Euler((0.0, -0.0, 0.0), 'XYZ')


Create a transform matrix from our Euler. Make it 4x4 so we can multiply it by the 4x4 object matrix.  Notice after the first multiplication we get the result as above.

&gt;&gt;&gt; R = Euler((x, y, z)).to_matrix().to_4x4()
&gt;&gt;&gt; ob.matrix_world = R @ ob.matrix_world
&gt;&gt;&gt; ob.rotation_euler
Euler((0.5235987901687622, 0.7853982448577881, 1.0471975803375244), 'XYZ')


and again

&gt;&gt;&gt; ob.matrix_world = R @ ob.matrix_world
&gt;&gt;&gt; ob.rotation_euler
Euler((1.6257474422454834, 0.4853370189666748, 2.580118417739868), 'XYZ')


Please note this is global rotation.  For local use ob.matrix_local instead.

Note: As pointed out by @lemon for Eulers could also use 

ob.rotation_euler.rotate(Euler((x, y, z)))


to transform  the local euler rotation from another.

For a lot of cases, the axis and angle is defined.  Eg make a 4x4 rotation matrix of 30 degrees around the axis (1, 1, 1)

R = Matrix.Rotation(radians(30), 4, (1, 1, 1))


Our R from above as an axis / angle rotation

&gt;&gt;&gt; R.to_quaternion().to_axis_angle()
(Vector((0.03912381827831268, 0.7727739214897156, 0.6334742903709412)), 1.2104885578155518)


is the equivalent of rotating 69.356 degrees around axis (0.03912381827831268, 0.7727739214897156, 0.6334742903709412)