�  Abstract  � Although there are several veterinary diagnostic laboratories in different executive, educational and research organizations in Iran, no comprehensive report exists on their roles in diagnosis of animal diseases. Therefore both laboratory workers and animal owner’s are not well aware of laboratory’s important in diagnosis of animal diseases.   Since most of parasitic infections produce subclinical form of the disease, it is necessary to ask veterinary diagnostic laboratories for accurate diagnosis and subsequently perform correct treatment, control, prevention and eradication programs. Parasitological, serological and molecular methods can be used, to achieve these goals. Therefore correct sampling and transferring samples to the laboratory, type of materials and preservative solutions, false negative and positive results and… may severely affect the results of the test done hence special care should be taken into consideration.    As a rule, the target in parasitological methods is to observe infectious organism in the received materials such as: feces, blood, urine, sputum, cutaneous lesions, eyes secretions, brain, spinal cord tissues and biopsy taken from different organs . Through parasitological methods and examination these materials, sporadic diagnosis and study of epidemiology, ecology, geographical distribution, zoonotic importance and….of parasitic infections become feasible. The aim of this paper (part I), is to present different methods commonly used for diagnosis of helminth infections.