The study was conducted to analyze the items about the ability of high school students diagram representation and obtain Item Curve Characteristic. Grid test instruments are compiled based on competencies and indicators of diagram representation which are then used to compile items. The test instrument consisted of five items and was validated by measurement experts, physics education experts, material experts and practitioners. The validated instrument was piloted in 296 students from three schools in Batang Regency, Central Java. Politomous data were analyzed using the QUEST program for classical analysis and Item Response Theory for modern analysis based on Partial Credit Model using PARSCALE program. The results of item test data analysis showed that the whole item was fit with Partial Credit Model. Reliability of the test instrument is based on internal consistency of 0.66 and the level of difficulty of the items is in the range 0.84 to 1.10. The information function and Standard Error Measurement show that test items are developed reliably to measure the ability of students' diagrams with the medium category, namely -1.2 < ? < +2.2 logit.