With the progress of the research in Quantum Computation and other unconventional forms of computations（e.g.DNA computation）

the idea that one can just look physical procedures themselves as computations

or more precisely

one can solve a problem by substituting specific physical procedures Turing machines has also been developed.However

because of the obscurity in its fundamental concepts

this theory so far actually remains quite ambiguous and makes people hard to analyze. In this paper

we unify the forms of input/output

strictly describe the three crucial parts of all physical experiments: initialization

evolution and measurement

and then

introduce a formal definition of physical computation（including the deterministic version and the probabilistic version）.Based on this

the concept of physical complexity is given

which includes energy and mass as two new fundamental resources in addition to the classical ones: time and space.So the complexity qualifies the amount of these 4 resources with respect to the length of input.As examples

we have analyzed DNA computation

Quantum computation and a famous example-Calculating average of three numbers using statistical physics

due to Pitowsky. Besides

the issues about the computability of some classical non-computable functions under physical computation are also discussed in this paper.We conclude

that all of the examples discussed in this paper fail to outdo universal Turing machine in computability

considering the infinite resources they cost before obtaining the correct answer.