This study aimed to examine the benefits and drawbacks of both solid phase and liquid phase radioimmunoassays (RIA) used to quantify C-peptide in human serum. Three fundamental reagents were made. The first RIA tracer was 125I-C-peptide. Its indirect preparation utilized chloramine-T and tyrosine methyl ester (TME). Using an HPLC column, the tracer was cleaned. The second one was the polyclonal antibodies which were produced in other work previously. The coated process using coated tube was carried out. Thirdly, using highly purified C-peptide, a set of standards was created. A liquid phase system radioimmunoassay (RIA) was created to measure C-peptide levels in human serum. Numerous elements were investigated, including cross reaction, specific and nonspecific binding, sample volume, temperature influence, incubation time, and validity period. Different samples of human serum were determined using both methods. The statistical study showed strong connections between the outcomes of the two procedures, but there were minor differences. The solid phase has many advantages than other one. A high nonspecific binding and a low specific binding percent were observed. A high binding percent and the low cost are the main advantages in Liquid phase system.